feat: WiFi beacon price discovery via Vendor IE (two-board verified)

Price discovery allows TollGate ESP32 boards to advertise their per-step
price via WiFi Vendor-Specific Information Elements (OUI 0xC0FFEE) in
beacon and probe response frames. Nearby boards passively scan and build
a market view of competing TollGates without requiring internet access.

Features:
- beacon_price.c/h: 26-byte packed Vendor IE payload (price, step, metric,
  mint_hash, geohash, npub_hash), injected via esp_wifi_set_vendor_ie()
- market.c/h: Passive WiFi scan receiver, vendor IE callback parsing,
  BSSID-correlated market entries, effective price ranking
- GET /market API endpoint: JSON market snapshot with discovered entries
- AP-only services: beacon + market + API start on WIFI_EVENT_AP_START,
  independent of STA connectivity
- STA reconnect fix: 2s delay between retries creates scan windows;
  s_sta_connecting guard prevents double-connect
- write-config-ap-only-a/b Makefile targets for STA-less testing
- market_tick() in main loop, client price comparison logging

Hardware verified: both boards discover each other via Vendor IE beacons.
Board A sees TollGate-C0E9CA (RSSI=-30), Board B sees TollGate-B96D80
(RSSI=-25). test-market.mjs: 9/9, test-price-discovery.mjs: 7/7 per board.

Unit tests: 45 new assertions across test_beacon_price (28) and test_market
(17). All 15 test suites pass. ESP-IDF build clean for ESP32-S3.

1 files changed, 44 insertions, 0 deletions

diff --git a/main/beacon_price.h b/main/beacon_price.h
new file mode 100644
index 0000000..cb2eb5b
--- /dev/null
+++ b/main/beacon_price.h
@@ -0,0 +1,44 @@
+#ifndef BEACON_PRICE_H
+#define BEACON_PRICE_H
+
+#include "esp_err.h"
+#include <stdint.h>
+#include <stdbool.h>
+
+#define TOLLGATE_OUI_0       0xC0
+#define TOLLGATE_OUI_1       0xFF
+#define TOLLGATE_OUI_2       0xEE
+#define TOLLGATE_IE_TYPE     0x01
+#define TOLLGATE_IE_VERSION  1
+
+#define TOLLGATE_IE_GEOHASH_MAX 9
+
+typedef struct __attribute__((packed)) {
+    uint8_t version;
+    uint8_t metric;
+    uint16_t price_per_step;
+    uint32_t step_size;
+    uint8_t mint_hash[4];
+    uint8_t geohash_len;
+    char geohash[TOLLGATE_IE_GEOHASH_MAX];
+    uint8_t npub_hash[4];
+} tollgate_price_payload_t;
+
+#define TOLLGATE_IE_PAYLOAD_SIZE sizeof(tollgate_price_payload_t)
+#define TOLLGATE_IE_TOTAL_SIZE   (6 + TOLLGATE_IE_PAYLOAD_SIZE)
+
+typedef struct __attribute__((packed)) {
+    uint8_t element_id;
+    uint8_t length;
+    uint8_t vendor_oui[3];
+    uint8_t vendor_oui_type;
+    tollgate_price_payload_t payload;
+} tollgate_price_ie_t;
+
+esp_err_t beacon_price_start(void);
+esp_err_t beacon_price_stop(void);
+void beacon_price_build_ie(tollgate_price_ie_t *ie);
+void beacon_price_hash_mint(const char *mint_url, uint8_t hash_out[4]);
+void beacon_price_hash_npub(const char *npub_hex, uint8_t hash_out[4]);
+
+#endif