docs: add tollgate_core component architecture design plan

Design document for extracting core TollGate modules (cashu, dns, firewall,
session) into a reusable ESP-IDF component consumed by esp-miner via the
IDF Component Manager.

Includes platform interface specification, wallet integration design,
dependency analysis, and phased implementation checklist.

Refs: feature/tollgate-core-component

1 files changed, 380 insertions, 0 deletions

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+# TollGate Core Component: Architecture Design
+
+## Goal
+
+Maintain all TollGate business logic in `esp32-tollgate` as a reusable ESP-IDF
+component (`tollgate_core`), and consume it in `esp-miner` (BitAxe) via the
+**IDF Component Manager**. No code duplication, no manual sync.
+
+## Current State (Pre-Refactoring)
+
+All TollGate modules live flat in `esp32-tollgate/main/`:
+
+```
+esp32-tollgate/main/
+  cashu.c / cashu.h
+  dns_server.c / dns_server.h
+  firewall.c / firewall.h
+  session.c / session.h
+  tollgate_api.c / tollgate_api.h
+  tollgate_client.c / tollgate_client.h
+  config.c / config.h
+  ...
+```
+
+The ESP-Miner port (`esp-miner/main/tollgate_*.c`) is a manual copy with edits:
+stripped prefixes (`cashu_` → `tollgate_cashu_`), NVS config instead of
+`config.h` singleton, removed wallet integration, moved cross-module wiring.
+
+### Shared Code by Module
+
+| Module | Shared % | Key Differences |
+|--------|----------|-----------------|
+| cashu | 73% | Config access, mint check parameterized |
+| dns_server | 74% | Minor logic reorder, logging stripped |
+| firewall | 94% | Cross-module DNS notification moved |
+| session | 79% | Bytes metric stripped, DNS notification added |
+| tollgate_api vs tollgate.c | 13% | Full rewrite (HTTP server vs library API) |
+| tollgate_client | 0% | No ESP-Miner equivalent |
+
+## Target Architecture
+
+### Directory Layout (in `esp32-tollgate`)
+
+```
+esp32-tollgate/
+  components/
+    tollgate_core/                    ← shared ESP-IDF component
+      CMakeLists.txt
+      idf_component.yml              ← component metadata for IDF Component Manager
+      include/
+        tollgate_core.h              ← public API
+        tollgate_platform.h          ← platform interface (config/state callbacks)
+      src/
+        tollgate_core_cashu.c        ← from main/cashu.c
+        tollgate_core_cashu.h
+        tollgate_core_dns.c          ← from main/dns_server.c
+        tollgate_core_dns.h
+        tollgate_core_firewall.c     ← from main/firewall.c
+        tollgate_core_firewall.h
+        tollgate_core_session.c      ← from main/session.c
+        tollgate_core_session.h
+    nucula_lib/                       ← stays as-is (git submodule + wrapper)
+      CMakeLists.txt
+      nucula_wallet.cpp / .h
+  main/
+    tollgate_platform.c               ← standalone impl of tollgate_platform.h
+    tollgate_api.c / .h               ← standalone HTTP server (unchanged)
+    tollgate_client.c / .h            ← standalone client mode (unchanged)
+    config.c / config.h               ← standalone config (unchanged)
+    ...
+```
+
+### How ESP-Miner Consumes It
+
+In `esp-miner/main/idf_component.yml`:
+
+```yaml
+dependencies:
+  tollgate/core:
+    git: https://github.com/<user>/esp32-tollgate.git
+    path: components/tollgate_core
+```
+
+ESP-Miner provides only:
+
+```
+esp-miner/main/
+  tollgate_platform.c               ← implements tollgate_platform.h (NVS config)
+  tollgate.c / .h                    ← ESP-Miner orchestrator (owner detection, WiFi events)
+  tollgate_page.html                 ← captive portal payment UI
+  lwip_tollgate_hooks.h              ← LWIP hook (stays in esp-miner)
+  http_server.c                      ← modified to call tollgate_core API
+```
+
+### Why IDF Component Manager (not submodule)
+
+| Aspect | IDF Component Manager | Git Submodule |
+|--------|----------------------|---------------|
+| What's downloaded | Only `components/tollgate_core/` | Entire `esp32-tollgate` repo |
+| Update mechanism | Modify version in yml, rebuild | Manual `git submodule update` |
+| Transitive deps | Automatic (nucula_lib resolved) | Must manage manually |
+| CI/CD | Automatic on `idf.py build` | Needs `--recursive` clone |
+| Offline after first build | Yes (cached in managed_components) | Yes |
+| Contributor friction | Low (automatic) | Moderate (forgot --recursive) |
+
+ESP-Miner never reaches into tollgate_core's source tree. It calls a clean API
+and provides a platform implementation. This is exactly the "packaged API
+consumption" pattern the Component Manager is designed for.
+
+### Why Git Submodule for nucula (not Component Manager)
+
+nucula is consumed differently — it's a **raw source integration**:
+
+```cmake
+# nucula_lib/CMakeLists.txt reaches INTO the submodule and cherry-picks files:
+set(NUCULA_SRC ${CMAKE_CURRENT_SOURCE_DIR}/../../nucula_src/main)
+idf_component_register(
+    SRCS "nucula_wallet.cpp"
+         "${NUCULA_SRC}/crypto.c"       # cherry-picked
+         "${NUCULA_SRC}/wallet.cpp"     # cherry-picked
+         "${NUCULA_SRC}/cashu_json.cpp" # cherry-picked (6 of ~20 files)
+         "${NUCULA_SRC}/nut10.cpp"
+         "${NUCULA_SRC}/hex.c"
+         "${NUCULA_SRC}/http.c"
+    ...
+)
+```
+
+The Component Manager downloads packaged components — you get everything or
+nothing. You can't say "give me this component but only compile these 6 files
+from it." A git submodule gives you the raw source tree on disk, which is what
+cherry-picking requires.
+
+**Principle:** Need to reach into source tree and pick files? → Submodule.
+Only need a clean API? → Component Manager.
+
+### The Platform Interface
+
+```c
+// components/tollgate_core/include/tollgate_platform.h
+
+#ifndef TOLLGATE_PLATFORM_H
+#define TOLLGATE_PLATFORM_H
+
+#include <stdint.h>
+#include <stdbool.h>
+
+typedef struct {
+    // Config access (each project implements its own storage)
+    uint16_t     (*get_price_sats)(void);
+    int32_t      (*get_step_ms)(void);
+    const char * (*get_mint_url)(void);
+    const char * (*get_metric)(void);         // "milliseconds" or "bytes"
+    int32_t      (*get_step_bytes)(void);
+
+    // Time source
+    int64_t      (*get_time_ms)(void);
+
+    // Wallet integration: called after proofs verified, before session create
+    // Return true to proceed, false to reject payment
+    // Can be NULL (accepts payment without spending proofs — double-spend risk)
+    bool         (*spend_proofs)(const char *raw_token_json);
+} tollgate_platform_t;
+
+#endif
+```
+
+**Standalone implementation** (`main/tollgate_platform.c`):
+- Reads from `tollgate_config_get()` singleton (SPIFFS-backed)
+- `spend_proofs` calls `nucula_wallet_receive()` to swap proofs at the mint
+
+**ESP-Miner implementation** (`main/tollgate_platform.c`):
+- Reads from `nvs_config_get_*()` (NVS flash)
+- `spend_proofs` is initially NULL (Phase 1: accept without spending)
+- Later: calls nucula_wallet_receive when wallet component is integrated
+
+### Wallet Integration: The Double-Spend Problem
+
+The `spend_proofs` hook exists because of a real security gap:
+
+```
+Client sends Cashu token
+    │
+    ▼
+cashu_decode_token()          ← extract proofs
+    │
+    ▼
+cashu_check_proof_states()    ← HTTP POST to mint /v1/checkstate: "unspent?"
+    │
+    ▼
+spend_proofs()                ← THE CRITICAL STEP
+    │                              standalone: nucula_wallet_receive() → swap at mint
+    │                              esp-miner: NULL → skipped (double-spend window)
+    ▼
+session_create()              ← grant client access
+```
+
+Without `spend_proofs`, a client can replay the same token on multiple devices.
+Both check "unspent?" → both say yes → both grant access. The swap step marks
+proofs as spent at the mint, closing the window.
+
+ESP-Miner accepts this risk initially. When `spend_proofs` is NULL, the
+component logs a warning. Phase 2 of ESP-Miner integration adds nucula and
+implements the hook.
+
+### Cross-Module Wiring (Internal to tollgate_core)
+
+The `session → firewall → dns_server` notification chain stays internal:
+
+```
+tollgate_core_session_create()
+  → tollgate_core_firewall_grant(ip)
+      → tollgate_core_dns_set_authenticated(ip, true)
+
+tollgate_core_session_revoke()
+  → tollgate_core_firewall_revoke(ip)
+      → tollgate_core_dns_set_authenticated(ip, false)
+```
+
+Consumers never see this. They call `tollgate_core_process_payment()` and
+`tollgate_core_tick()`. The internal wiring is an implementation detail.
+
+### Full Dependency Graph
+
+```
+esp-miner
+  └── IDF Component Manager → tollgate_core (API-level boundary)
+        ├── CMakeLists.txt REQUIRES: nucula_lib
+        └── Platform: esp-miner provides tollgate_platform_t (NVS-backed)
+
+esp32-tollgate (standalone)
+  └── tollgate_core (local component, same repo)
+        ├── CMakeLists.txt REQUIRES: nucula_lib
+        └── Platform: main/tollgate_platform.c (config singleton-backed)
+
+nucula_lib (local component in esp32-tollgate)
+  └── cherry-picks source files from nucula_src/ (git submodule → zeugmaster/nucula)
+```
+
+### Dependency Chain for IDF Component Manager
+
+When `esp-miner` declares:
+
+```yaml
+dependencies:
+  tollgate/core:
+    git: https://github.com/<user>/esp32-tollgate.git
+    path: components/tollgate_core
+```
+
+The Component Manager:
+1. Clones `esp32-tollgate` (or fetches the component archive)
+2. Reads `tollgate_core/idf_component.yml` → finds dependency on `nucula_lib`
+3. Since `nucula_lib` is a sibling component in the same repo, resolves it
+   from the same clone
+4. Downloads into `managed_components/`
+5. `nucula_lib` depends on `secp256k1` (local component) and `nucula_src`
+   (submodule) — these must be available within the cloned repo
+
+**Note:** The git submodule within `nucula_src` needs verification. The IDF
+Component Manager may or may not initialize submodules within a git-sourced
+dependency. This needs testing. If it doesn't, `nucula_lib` may need to bundle
+the required nucula source files directly instead of referencing a submodule.
+
+## Blocking Dependencies
+
+This refactoring **must not proceed** until these branches land on master:
+
+| Branch | Blocking Files | Status |
+|--------|---------------|--------|
+| `feature/multi-mint-support` | `cashu.c`, `tollgate_api.c`, `main/CMakeLists.txt`, `nucula_wallet.cpp/h`, `captive_portal.c`, `mint_health.c/h`, `config.c/h` | **In progress** |
+| `feature/price-discovery` | `tollgate_api.c`, `tollgate_client.c`, `main/CMakeLists.txt`, `config.c/h`, `beacon_price.c/h`, `market.c/h` | **In progress** |
+| `feature/cvm-integration` | Same commit as master — no new changes | **Merged already** |
+
+**Specific conflicts if we refactor now:**
+- Moving `cashu.c` → `tollgate_core_cashu.c` while multi-mint modifies `cashu.c`
+- Moving `dns_server.c` while price-discovery may touch it
+- Modifying `main/CMakeLists.txt` (remove SRCS) while all branches modify it
+- Modifying `tollgate_api.c` call sites while multi-mint and price-discovery modify it
+
+## Refactoring Plan (After Blocking PRs Merge)
+
+### Phase 0: Prerequisites
+
+- [ ] All blocking PRs merged to master
+- [ ] This branch rebased onto latest master
+- [ ] Full build passes on master
+
+### Phase 1: Create Component Skeleton
+
+- [ ] Create `components/tollgate_core/` directory structure
+- [ ] Create `components/tollgate_core/include/tollgate_core.h` (public API)
+- [ ] Create `components/tollgate_core/include/tollgate_platform.h` (platform interface)
+- [ ] Create `components/tollgate_core/idf_component.yml` (component metadata)
+- [ ] Create `components/tollgate_core/CMakeLists.txt` (register component, REQUIRES nucula_lib)
+- [ ] Verify empty component builds without errors
+
+### Phase 2: Move Core Modules (one at a time, build after each)
+
+- [ ] Copy `main/cashu.c/h` → `components/tollgate_core/src/tollgate_core_cashu.c/h`
+  - [ ] Rename functions: `cashu_*` → `tollgate_core_cashu_*`
+  - [ ] Replace `tollgate_config_get()` calls with platform callbacks
+  - [ ] Remove direct `config.h` include
+  - [ ] Build and verify
+- [ ] Copy `main/dns_server.c/h` → `components/tollgate_core/src/tollgate_core_dns.c/h`
+  - [ ] Rename functions: `dns_server_*` → `tollgate_core_dns_*`
+  - [ ] No platform dependencies (pure LWIP) — should be clean copy
+  - [ ] Build and verify
+- [ ] Copy `main/firewall.c/h` → `components/tollgate_core/src/tollgate_core_firewall.c/h`
+  - [ ] Rename functions: `firewall_*` → `tollgate_core_firewall_*`
+  - [ ] Internalize `dns_set_authenticated` calls (keep within component)
+  - [ ] Remove `dns_server.h` external dependency
+  - [ ] Build and verify
+- [ ] Copy `main/session.c/h` → `components/tollgate_core/src/tollgate_core_session.c/h`
+  - [ ] Rename functions: `session_*` → `tollgate_core_session_*`
+  - [ ] Replace `config.h` calls with platform callbacks for metric check
+  - [ ] Internalize firewall notification (already calls firewall directly)
+  - [ ] Support both time and bytes metrics (portable, not stripped)
+  - [ ] Build and verify
+
+### Phase 3: Wire Component API
+
+- [ ] Implement `tollgate_core_init(const tollgate_platform_t *platform)` — stores platform, inits all sub-modules
+- [ ] Implement `tollgate_core_process_payment(ip, token)` — decode → verify → spend → create session
+- [ ] Implement `tollgate_core_client_connected(mac, ip)` — owner detection + firewall check
+- [ ] Implement `tollgate_core_client_disconnected(mac)` — session cleanup + owner reassign
+- [ ] Implement `tollgate_core_tick()` — session expiry check
+- [ ] Implement `tollgate_core_get_status_json()` — JSON status
+- [ ] Implement `tollgate_core_get_config_json()` — JSON config (via platform)
+- [ ] Build and verify standalone
+
+### Phase 4: Standalone Platform Implementation
+
+- [ ] Create `main/tollgate_platform.c` implementing `tollgate_platform_t`
+  - [ ] `get_price_sats` → `tollgate_config_get()->price_per_step`
+  - [ ] `get_step_ms` → `tollgate_config_get()->step_size`
+  - [ ] `get_mint_url` → `tollgate_config_get()->mint_url`
+  - [ ] `get_metric` → `tollgate_config_get()->metric`
+  - [ ] `get_step_bytes` → `tollgate_config_get()->step_bytes`
+  - [ ] `get_time_ms` → `xTaskGetTickCount() * portTICK_PERIOD_MS`
+  - [ ] `spend_proofs` → `nucula_wallet_receive()`
+- [ ] Update `main/tollgate_api.c` to call `tollgate_core_*` instead of direct module calls
+- [ ] Update `main/tollgate_main.c` init sequence
+- [ ] Remove old `main/cashu.c`, `main/dns_server.c`, `main/firewall.c`, `main/session.c`
+- [ ] Update `main/CMakeLists.txt` (remove old SRCS, add `tollgate_platform.c`)
+- [ ] Full standalone build + test
+
+### Phase 5: ESP-Miner Integration
+
+- [ ] Update `esp-miner/main/idf_component.yml` to add tollgate_core dependency
+- [ ] Create `esp-miner/main/tollgate_platform.c` implementing `tollgate_platform_t`
+  - [ ] Config reads from NVS (`nvs_config_get_*`)
+  - [ ] `spend_proofs` = NULL initially (Phase 1: accept without spending)
+- [ ] Update `esp-miner/main/tollgate.c` to call `tollgate_core_*` API
+- [ ] Remove `esp-miner/main/tollgate_cashu.c`, `tollgate_dns.c`, `tollgate_firewall.c`, `tollgate_session.c`
+- [ ] Update `esp-miner/main/CMakeLists.txt` (remove old SRCS)
+- [ ] Full ESP-Miner build + test
+
+### Phase 6: Verify Component Manager Flow
+
+- [ ] Remove local `managed_components/` if present
+- [ ] Run `idf.py reconfigure` in esp-miner — verify Component Manager downloads tollgate_core
+- [ ] Run `idf.py build` — verify transitive dependency resolution (nucula_lib + nucula_src)
+- [ ] Test that submodule within nucula_src is properly initialized by Component Manager
+- [ ] If submodule init fails: bundle nucula source files directly in nucula_lib instead
+
+### Phase 7: Documentation and Cleanup
+
+- [ ] Update `esp-miner/main/idf_component.yml` with correct git URL
+- [ ] Update `esp-miner/TOLLGATE_PR_PLAN.md` to reflect component-based architecture
+- [ ] Add `docs/` to `tollgate_core` with integration guide for new consumers
+- [ ] Update `esp-miner/TOLLGATE_CHECKLIST.md`
+- [ ] Verify both projects build clean from scratch
+
+## Open Questions
+
+- [ ] Does the IDF Component Manager initialize git submodules within git-sourced dependencies?
+- [ ] Should tollgate_core publish to the ESP Component Registry (public) or stay git-only?
+- [ ] What versioning scheme for tollgate_core? (semver tags in esp32-tollgate?)
+- [ ] Should `tollgate_client.c` (client mode) eventually move into tollgate_core?