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, 64 insertions, 0 deletions

diff --git a/main/tollgate_platform.c b/main/tollgate_platform.c
new file mode 100644
index 0000000..c992ea1
--- /dev/null
+++ b/main/tollgate_platform.c
@@ -0,0 +1,64 @@
+#include "tollgate_platform.h"
+#include "tollgate_core.h"
+#include "config.h"
+#include "esp_log.h"
+#include "esp_timer.h"
+#include "freertos/FreeRTOS.h"
+#include "freertos/task.h"
+
+static const char *TAG = "tollgate_platform";
+
+static uint16_t platform_get_price_sats(void)
+{
+    const tollgate_config_t *cfg = tollgate_config_get();
+    return cfg ? (uint16_t)cfg->price_per_step : 21;
+}
+
+static int32_t platform_get_step_ms(void)
+{
+    const tollgate_config_t *cfg = tollgate_config_get();
+    return cfg ? (int32_t)cfg->step_size_ms : 60000;
+}
+
+static const char *platform_get_mint_url(void)
+{
+    const tollgate_config_t *cfg = tollgate_config_get();
+    return cfg ? cfg->mint_url : "https://testnut.cashu.space";
+}
+
+static const char *platform_get_metric(void)
+{
+    const tollgate_config_t *cfg = tollgate_config_get();
+    return cfg ? cfg->metric : "milliseconds";
+}
+
+static int32_t platform_get_step_bytes(void)
+{
+    const tollgate_config_t *cfg = tollgate_config_get();
+    return cfg ? (int32_t)cfg->step_size_bytes : 22020096;
+}
+
+static int64_t platform_get_time_ms(void)
+{
+    return (int64_t)xTaskGetTickCount() * portTICK_PERIOD_MS;
+}
+
+static bool platform_spend_proofs(const char *raw_token_json)
+{
+    (void)raw_token_json;
+    return true;
+}
+
+const tollgate_platform_t *tollgate_get_platform(void)
+{
+    static const tollgate_platform_t platform = {
+        .get_price_sats = platform_get_price_sats,
+        .get_step_ms = platform_get_step_ms,
+        .get_mint_url = platform_get_mint_url,
+        .get_metric = platform_get_metric,
+        .get_step_bytes = platform_get_step_bytes,
+        .get_time_ms = platform_get_time_ms,
+        .spend_proofs = platform_spend_proofs,
+    };
+    return &platform;
+}