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, 40 insertions, 0 deletions

diff --git a/main/market.h b/main/market.h
new file mode 100644
index 0000000..6dbf43c
--- /dev/null
+++ b/main/market.h
@@ -0,0 +1,40 @@
+#ifndef MARKET_H
+#define MARKET_H
+
+#include "beacon_price.h"
+#include "esp_wifi.h"
+#include "esp_err.h"
+#include <stdint.h>
+#include <stdbool.h>
+
+#define MARKET_MAX_ENTRIES 10
+
+typedef struct {
+    uint8_t bssid[6];
+    char ssid[33];
+    int8_t rssi;
+    uint16_t price_per_step;
+    uint32_t step_size;
+    uint8_t metric;
+    uint8_t mint_hash[4];
+    uint8_t npub_hash[4];
+    char geohash[TOLLGATE_IE_GEOHASH_MAX + 1];
+    int64_t discovered_ms;
+    bool valid;
+} market_entry_t;
+
+typedef struct {
+    market_entry_t entries[MARKET_MAX_ENTRIES];
+    int count;
+    int64_t last_scan_ms;
+    bool scanning;
+    int consecutive_failures;
+} market_t;
+
+esp_err_t market_init(void);
+void market_tick(void);
+const market_t *market_get(void);
+int market_find_cheapest(void);
+void market_parse_vendor_ie(const uint8_t sa[6], const vendor_ie_data_t *ie, int rssi);
+
+#endif