Handle matchmaking socket close codes per API contract (#4595)

## What

Implements the close-code contract from the matchmaking API handoff (API
PR #419) in the matchmaking modal, plus two runnable integration
harnesses. Previously `onclose` only logged, so an API deploy while
queued left the player on the "searching" spinner forever (the queue is
in-memory on the API worker).

| Close | Behavior |
| --- | --- |
| 1008 `Invalid session` | Reconnect and re-send `join` —
`getPlayToken()` refreshes an expired token internally, so the rejoin
carries a fresh JWT |
| 1000 `Replaced by newer connection` | Another tab/window took the
queue slot: show a message (new `matchmaking_modal.replaced` string) and
stop. No retry |
| Any other close before assignment | Server restart/deploy: reconnect
and re-send `join`, with exponential backoff (1s doubling to a 15s cap)
|

Intentional closes (user backs out of the modal, assignment received)
don't reconnect. A pending 2s join timer from a previous socket is
cleared before each reconnect so it can't fire on the new socket.

## Test harnesses (`tests/matchmaking/`)

- **`npm run test:matchmaking`** (contained): drives the real modal in a
headless browser against a fake in-process matchmaking server speaking
the documented protocol; covers the whole close-code table over real
WebSockets. Needs only `npm run dev`.
- **`npm run test:matchmaking:e2e`**: real integration against the API
worker on `localhost:8787` — two browser players join the real queue,
the dev game server receives the checkin assignment, and both clients
end up in the same created game.

## Why now

This is required by the matchmaking API's client contract independent of
the upcoming 2v2 work ("clients must already handle unexpected close →
reconnect and rejoin"). The rest of the 2v2 integration is planned for
the next version.

## Verification

- Contained harness: 8/8 checks pass (join, 1006 reconnect+rejoin, 1008
rejoin with fresh token, assignment, replaced → message + no retry,
intentional close → no retry/no message).
- E2E harness against a local worker: 3/3 — both players matched into
the same game, game created via checkin and joinable by both.
- `npm test` (all 2,046 pass), `npx tsc --noEmit`, ESLint clean.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

tests/matchmaking/util.mjs

@@ -0,0 +1,44 @@
+// Shared helpers for the matchmaking harnesses (contained + e2e).
+
+export async function waitFor(
+  fn,
+  { timeoutMs = 10000, intervalMs = 200, label = "condition" } = {},
+) {
+  const deadline = Date.now() + timeoutMs;
+  for (;;) {
+    const value = await fn();
+    if (value) return value;
+    if (Date.now() > deadline) {
+      throw new Error(`Timed out after ${timeoutMs}ms waiting for: ${label}`);
+    }
+    await new Promise((r) => setTimeout(r, intervalMs));
+  }
+}
+
+// Any HTTP response (including 404) counts as "up"; only a network error
+// (nothing listening) counts as down.
+export async function isUp(url) {
+  try {
+    await fetch(url, { signal: AbortSignal.timeout(2000) });
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+export function makeChecker() {
+  const results = [];
+  return {
+    check(name, cond) {
+      results.push({ name, pass: !!cond });
+      console.log(`${cond ? "PASS" : "FAIL"}  ${name}`);
+    },
+    finish() {
+      const failed = results.filter((r) => !r.pass);
+      console.log(
+        `\n${results.length - failed.length}/${results.length} checks passed`,
+      );
+      process.exit(failed.length > 0 ? 1 : 0);
+    },
+  };
+}