feat: create tollgate_core standalone C library + tollgate_esp IDF component

Phase 1-4: Extract core business logic into reusable library

tollgate_core/ — standalone C library (zero ESP-IDF deps):
  - tollgate_core.c — orchestrator (init, process_payment, process_share, tick)
  - tollgate_cashu.c — V3 token decode, checkstate via platform callback, allotment math
  - tollgate_session.c — session lifecycle state machine (create/find/revoke/extend)
  - tollgate_firewall.c — client allowlist, NAT filter, packet filter
  - tollgate_mining.c — hashprice, nbits→difficulty, shares-to-allotment, client stats

components/tollgate_esp/ — ESP-IDF component wrapper:
  - tollgate_esp_platform.c — implements tollgate_platform_t using ESP-IDF APIs
  - CMakeLists.txt — compiles tollgate_core + ESP wrapper, links to ESP-IDF
  - idf_component.yml — for IDF Component Registry

Platform interface (tollgate_platform.h):
  - Config access: price, step_size, mint_url, metric
  - Time: get_time_ms (via FreeRTOS ticks on ESP)
  - Logging: log_info/warn/error (via ESP_LOG on ESP)
  - Wallet: receive/send/balance (via nucula on ESP)
  - HTTP: http_post (via esp_http_client on ESP)
  - WiFi/MAC: mac_for_ip, napt_enable
  - Mining: stratum config, hashprice override

Build verification:
  - tollgate_core compiles standalone: cmake + make (gcc, host)
  - tollgate_esp + ESP-IDF full build: passes (idf.py build)
  - Unit tests: 22/22 pass (make test-unit)
  - No behavioral change to existing main/ code yet (Phase 5)

1 files changed, 0 insertions, 0 deletions

diff --git a/tests/unit/test_tollgate_client_mining b/tests/unit/test_tollgate_client_mining
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