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, 14 insertions, 0 deletions

diff --git a/main/tollgate_client.c b/main/tollgate_client.c
index ac8dcfe..a81d16f 100644
--- a/main/tollgate_client.c
+++ b/main/tollgate_client.c
@@ -1,6 +1,7 @@
 #include "tollgate_client.h"
 #include "config.h"
 #include "nucula_wallet.h"
+#include "market.h"
 #include "esp_log.h"
 #include "esp_http_client.h"
 #include "esp_crt_bundle.h"
@@ -343,6 +344,19 @@ esp_err_t tollgate_client_on_sta_connected(const char *gw_ip_str)
     s_state = TG_CLIENT_PAID;
 
     ESP_LOGI(TAG, "upstream TollGate paid: %lldms allotment", (long long)allotment);
+
+    const market_t *mkt = market_get();
+    int cheapest = market_find_cheapest();
+    if (cheapest >= 0 && mkt->entries[cheapest].valid && mkt->entries[cheapest].ssid[0] != '\0') {
+        uint32_t upstream_step = s_discovery.step_size_ms > 0 ? s_discovery.step_size_ms : 1;
+        uint32_t upstream_eff = (uint32_t)s_discovery.price_per_step * 60000 / upstream_step;
+        uint32_t cheap_step = mkt->entries[cheapest].step_size > 0 ? mkt->entries[cheapest].step_size : 1;
+        uint32_t cheap_eff = (uint32_t)mkt->entries[cheapest].price_per_step * 60000 / cheap_step;
+        if (cheap_eff < upstream_eff) {
+            ESP_LOGW(TAG, "CHEAPER TOLLGATE AVAILABLE: %s at %lu sats/min vs upstream %lu sats/min",
+                     mkt->entries[cheapest].ssid, (unsigned long)cheap_eff, (unsigned long)upstream_eff);
+        }
+    }
     return ESP_OK;
 }