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, 9 insertions, 3 deletions

diff --git a/tests/unit/Makefile b/tests/unit/Makefile
index 6d13e4d..7bd3f1e 100644
--- a/tests/unit/Makefile
+++ b/tests/unit/Makefile
@@ -22,7 +22,7 @@ LDFLAGS := -lmbedcrypto -lcjson -lm
 
 SECP256K1_OBJ := secp256k1.o precomputed_ecmult.o precomputed_ecmult_gen.o
 
-TESTS := test_geohash test_identity test_nostr_event test_cashu test_session test_tollgate_client test_lnurl_pay test_lightning_payout test_mcp_handler test_nip04 test_cvm_server test_display test_negentropy_adapter
+TESTS := test_geohash test_identity test_nostr_event test_cashu test_session test_tollgate_client test_lnurl_pay test_lightning_payout test_mcp_handler test_nip04 test_cvm_server test_display test_negentropy_adapter test_beacon_price test_market
 
 .PHONY: all test clean $(TESTS)
 
@@ -63,8 +63,8 @@ test_cashu: test_cashu.c $(REPO_ROOT)/main/cashu.c
 test_session: test_session.c $(REPO_ROOT)/main/session.c $(REPO_ROOT)/main/cashu.c
         $(CC) $(CFLAGS) $< $(REPO_ROOT)/main/session.c $(REPO_ROOT)/main/cashu.c -o $@ $(LDFLAGS)
 
-test_tollgate_client: test_tollgate_client.c
-        $(CC) $(CFLAGS) $< -o $@ $(LDFLAGS)
+test_tollgate_client: test_tollgate_client.c $(REPO_ROOT)/main/market.c $(REPO_ROOT)/main/beacon_price.c
+        $(CC) $(CFLAGS) -I $(REPO_ROOT)/main $< $(REPO_ROOT)/main/market.c $(REPO_ROOT)/main/beacon_price.c -o $@ $(LDFLAGS)
 
 test_lnurl_pay: test_lnurl_pay.c
         $(CC) $(CFLAGS) $< -o $@ $(LDFLAGS)
@@ -87,5 +87,11 @@ test_display: test_display.c
 test_negentropy_adapter: test_negentropy_adapter.c
         $(CC) $(CFLAGS) $< -o $@ $(LDFLAGS)
 
+test_beacon_price: test_beacon_price.c $(REPO_ROOT)/main/beacon_price.c
+        $(CC) $(CFLAGS) -I $(REPO_ROOT)/main $< $(REPO_ROOT)/main/beacon_price.c -o $@ $(LDFLAGS)
+
+test_market: test_market.c $(REPO_ROOT)/main/market.c $(REPO_ROOT)/main/beacon_price.c
+        $(CC) $(CFLAGS) -I $(REPO_ROOT)/main $< $(REPO_ROOT)/main/market.c $(REPO_ROOT)/main/beacon_price.c -o $@ $(LDFLAGS)
+
 clean:
         rm -f $(TESTS) $(SECP256K1_OBJ)