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, 64 insertions, 0 deletions

diff --git a/tests/unit/stubs/esp_wifi.h b/tests/unit/stubs/esp_wifi.h
index 6aa5787..5eb14bf 100644
--- a/tests/unit/stubs/esp_wifi.h
+++ b/tests/unit/stubs/esp_wifi.h
@@ -2,6 +2,7 @@
 #define STUBS_ESP_WIFI_H
 
 #include <stdint.h>
+#include <stdbool.h>
 #include <string.h>
 #include "esp_err.h"
 
@@ -37,4 +38,67 @@ static inline esp_err_t esp_wifi_set_config(int ifx, const wifi_config_t *cfg) {
 static inline esp_err_t esp_wifi_set_mode(uint8_t mode) { (void)mode; return ESP_OK; }
 static inline esp_err_t esp_wifi_start(void) { return ESP_OK; }
 
+#define WIFI_VENDOR_IE_ELEMENT_ID 0xDD
+
+typedef enum {
+    WIFI_VND_IE_TYPE_BEACON,
+    WIFI_VND_IE_TYPE_PROBE_REQ,
+    WIFI_VND_IE_TYPE_PROBE_RESP,
+    WIFI_VND_IE_TYPE_ASSOC_REQ,
+    WIFI_VND_IE_TYPE_ASSOC_RESP,
+} wifi_vendor_ie_type_t;
+
+typedef enum {
+    WIFI_VND_IE_ID_0,
+    WIFI_VND_IE_ID_1,
+} wifi_vendor_ie_id_t;
+
+typedef struct {
+    uint8_t element_id;
+    uint8_t length;
+    uint8_t vendor_oui[3];
+    uint8_t vendor_oui_type;
+    uint8_t payload[0];
+} vendor_ie_data_t;
+
+typedef void (*esp_vendor_ie_cb_t)(void *ctx, wifi_vendor_ie_type_t type, const uint8_t sa[6], const vendor_ie_data_t *vnd_ie, int rssi);
+
+static inline esp_err_t esp_wifi_set_vendor_ie(bool enable, wifi_vendor_ie_type_t type, wifi_vendor_ie_id_t idx, const void *vnd_ie) { (void)enable; (void)type; (void)idx; (void)vnd_ie; return ESP_OK; }
+static inline esp_err_t esp_wifi_set_vendor_ie_cb(esp_vendor_ie_cb_t cb, void *ctx) { (void)cb; (void)ctx; return ESP_OK; }
+
+#define WIFI_SCAN_TYPE_PASSIVE 0
+
+typedef struct {
+    uint8_t bssid[6];
+    uint8_t ssid[33];
+    uint8_t primary;
+    int second;
+    int8_t rssi;
+    int authmode;
+} wifi_ap_record_t;
+
+typedef struct {
+    uint8_t *ssid;
+    uint8_t *bssid;
+    uint8_t channel;
+    bool show_hidden;
+    int scan_type;
+    union {
+        struct { int min; int max; } active;
+        int passive;
+    } scan_time;
+} wifi_scan_config_t;
+
+static inline esp_err_t esp_wifi_scan_start(const wifi_scan_config_t *cfg, bool block) { (void)cfg; (void)block; return ESP_OK; }
+static inline esp_err_t esp_wifi_scan_get_ap_num(uint16_t *n) { *n = 0; return ESP_OK; }
+static inline esp_err_t esp_wifi_scan_get_ap_records(uint16_t *n, wifi_ap_record_t *records) { (void)records; *n = 0; return ESP_OK; }
+
+#define WIFI_EVENT_SCAN_DONE 3
+
+typedef void *esp_event_handler_instance_t;
+typedef const char *esp_event_base_t;
+#define WIFI_EVENT "WIFI_EVENT"
+
+static inline esp_err_t esp_event_handler_instance_register(esp_event_base_t a, int32_t b, void *c, void *d, esp_event_handler_instance_t *e) { (void)a; (void)b; (void)c; (void)d; (void)e; return ESP_OK; }
+
 #endif