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, 6 insertions, 0 deletions

diff --git a/components/tollgate_core/idf_component.yml b/components/tollgate_core/idf_component.yml
new file mode 100644
index 0000000..112aa18
--- /dev/null
+++ b/components/tollgate_core/idf_component.yml
@@ -0,0 +1,6 @@
+version: "1.0.0"
+description: TollGate core component — Cashu payment processing, per-client DNS/firewall, session management for paid WiFi hotspots
+url: https://github.com/nicoulaj/esp32-tollgate
+dependencies:
+  idf:
+    version: ">=5.4.0"