feat(race-day): add warm-up transponder check before a heat

Race Day gets a 1-3 minute warm-up per heat with a green check per
driver when their transponder is seen, countdown announcements
(sv/en), quick transponder fix from the panel, and a start warning
that still allows starting when someone is missing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/browser_fake_decoder.js

@@ -12,8 +12,7 @@ export async function freePort() {
 
 // Accepts the WebSocket upgrade handshake so the frontend's real decoder-connect path
 // (src/decoder_runtime.js) reaches `connected: true` without a physical AMMC decoder.
-// No frames are sent or parsed: scenarios using this only need an established socket,
-// not lap data.
+// sendPassing() pushes a PASSING frame for scenarios that need the app to see a transponder.
 export async function startFakeDecoder() {
   const port = await freePort();
   const server = createServer();
@@ -40,6 +39,14 @@ export async function startFakeDecoder() {
     // handshake) to simulate a decoder/network interruption, while the server keeps
     // listening so the frontend's own reconnect logic has somewhere to reconnect to.
     dropConnections: () => { for (const socket of sockets) socket.destroy(); },
+    // Sends an AMMC-style PASSING message as an unmasked server->client WebSocket text frame.
+    sendPassing: (transponder) => {
+      const payload = Buffer.from(JSON.stringify({ msg: "PASSING", transponder: String(transponder), rtc_time: new Date().toISOString() }));
+      const header = payload.length < 126
+        ? Buffer.from([0x81, payload.length])
+        : Buffer.from([0x81, 126, payload.length >> 8, payload.length & 0xff]);
+      for (const socket of sockets) socket.write(Buffer.concat([header, payload]));
+    },
     close: () => new Promise(resolve => {
       for (const socket of sockets) socket.destroy();
       server.close(() => resolve());