feat: extract tollgate_core ESP-IDF component

Extract shared TollGate business logic into a reusable ESP-IDF component
that can be consumed by esp-miner via the IDF Component Manager.

Component structure:
  components/tollgate_core/
    include/tollgate_core.h       — public API
    include/tollgate_platform.h   — platform interface (config callbacks)
    src/tollgate_core.c           — orchestrator (init, payment, tick, owner)
    src/tollgate_core_cashu.c/h   — Cashu V3 token decode/verify
    src/tollgate_core_dns.c/h     — per-client DNS hijack/forward
    src/tollgate_core_firewall.c/h — per-client NAT filter
    src/tollgate_core_session.c/h — session lifecycle

Key design decisions:
  - Platform interface pattern: consumers implement tollgate_platform_t
    (config getters + optional spend_proofs wallet hook)
  - Cross-module wiring (session→firewall→dns) stays internal
  - No direct config.h dependency — all config via platform callbacks
  - spend_proofs can be NULL (accepts payment without local wallet)

Standalone app updated:
  - main/tollgate_platform.c implements platform via config singleton
  - main/tollgate_main.c calls tollgate_core_init/tick/dns_start
  - main/tollgate_api.c routes payment through tollgate_core_process_payment
  - Removed cashu.c, dns_server.c, firewall.c, session.c from CMakeLists

Not yet build-verified — blocked on multi-mint, price-discovery,
cvm-integration, and display-fix branches merging to master.

1 files changed, 2 insertions, 4 deletions

diff --git a/main/CMakeLists.txt b/main/CMakeLists.txt
index 9b0fb1c..5f195c3 100644
--- a/main/CMakeLists.txt
+++ b/main/CMakeLists.txt
@@ -1,10 +1,7 @@
 idf_component_register(SRCS "tollgate_main.c"
                             "config.c"
-                            "dns_server.c"
+                            "tollgate_platform.c"
                             "captive_portal.c"
-                            "firewall.c"
-                            "cashu.c"
-                            "session.c"
                             "tollgate_api.c"
                             "identity.c"
                             "nostr_event.c"
@@ -22,4 +19,5 @@ idf_component_register(SRCS "tollgate_main.c"
                     REQUIRES esp_wifi esp_event esp_netif nvs_flash esp_http_server
                              lwip json esp_http_client mbedtls esp-tls log spiffs
                              nucula_lib secp256k1 axs15231b qrcode
+                             tollgate_core
                     PRIV_REQUIRES esp-tls)