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, 60 insertions, 0 deletions

diff --git a/tests/integration/test-market.mjs b/tests/integration/test-market.mjs
new file mode 100644
index 0000000..20f062f
--- /dev/null
+++ b/tests/integration/test-market.mjs
@@ -0,0 +1,60 @@
+import { execSync } from 'child_process';
+
+const API_URL = `http://${process.env.TOLLGATE_IP || '10.192.45.1'}:2121`;
+
+function run(cmd) {
+  try { return execSync(cmd, { encoding: 'utf8', timeout: 15000 }); }
+  catch (e) { return e.stdout || null; }
+}
+
+function runJson(cmd) {
+  const out = run(cmd);
+  try { return out ? JSON.parse(out) : null; }
+  catch { return null; }
+}
+
+let passed = 0, failed = 0;
+function assert(cond, msg) {
+  if (cond) { console.log(`  PASS: ${msg}`); passed++; }
+  else { console.log(`  FAIL: ${msg}`); failed++; }
+}
+
+console.log('=== test-market (GET /market) ===\n');
+
+console.log('--- /market endpoint responds ---');
+{
+  const data = runJson(`curl -s --connect-timeout 5 ${API_URL}/market`);
+  assert(data !== null, '/market returns valid JSON');
+  assert(typeof data.count === 'number', `count is number (got ${data?.count})`);
+  assert(Array.isArray(data.entries), 'entries is array');
+}
+
+console.log('\n--- /market entry structure ---');
+{
+  const data = runJson(`curl -s --connect-timeout 5 ${API_URL}/market`);
+  if (data && data.entries && data.entries.length > 0) {
+    const e = data.entries[0];
+    assert(typeof e.bssid === 'string', `bssid is string (got ${e.bssid})`);
+    assert(typeof e.ssid === 'string', `ssid is string (got ${e.ssid})`);
+    assert(typeof e.rssi === 'number', `rssi is number (got ${e.rssi})`);
+    assert(typeof e.price_per_step === 'number', `price_per_step is number (got ${e.price_per_step})`);
+    assert(typeof e.step_size === 'number', `step_size is number (got ${e.step_size})`);
+    assert(typeof e.metric === 'string', `metric is string (got ${e.metric})`);
+  } else {
+    console.log('  SKIP: no entries found (scan may not have run yet)');
+  }
+}
+
+console.log('\n--- /market with no discovered TollGates ---');
+{
+  const data = runJson(`curl -s --connect-timeout 5 ${API_URL}/market`);
+  if (data && data.count === 0) {
+    assert(data.entries.length === 0, 'empty entries array when count=0');
+    console.log('  INFO: no nearby TollGates discovered yet (expected if only one board)');
+  } else if (data && data.count > 0) {
+    console.log(`  INFO: ${data.count} nearby TollGate(s) discovered`);
+  }
+}
+
+console.log(`\n=== Results: ${passed} passed, ${failed} failed ===`);
+process.exit(failed > 0 ? 1 : 0);