feat(host-control-mode): explicit server capabilities for feature detection

Replace the implicit "hostPeerId present ⇒ feature supported" heuristic with an
explicit capabilities list the relay advertises in ROOM_DATA. Cleaner, self-
documenting, and the extensible hook for the planned co-host feature (owner
promotes guests to extra controllers) — a future 'co-host' capability + events
slot in without a protocol bump.

- shared: CAPABILITIES { HOST_CONTROL }.
- server: SERVER_CAPABILITIES advertised in ROOM_DATA.
- background: track serverCapabilities (empty against older relay), serverSupports(),
  thread hostControlSupported through GET_STATUS / GET_CONTROL_MODE / CONTROL_MODE.
- popup: gate the host-control card on the explicit capability instead of hostPeerId.

Backwards-compatible both ways: old relay omits the field → feature stays hidden
(no errors); old client ignores the field. WS test asserts ROOM_DATA advertises
the capability. Adds docs/host-control-mode-TESTING.md (beta-server setup, wss
caveat, two-new-clients note, verification checklist).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/test-server-ws.mjs

@@ -63,6 +63,16 @@ try {
     close();
     resetConnectionRate();
 
+    // --- Capabilities: ROOM_DATA advertises server features for client detection ---
+    const capClient = await c();
+    s(capClient, 'join_room', { roomId: 'cap-'+Date.now(), peerId: 'capp', protocolVersion: '1.0.0' });
+    const [capEv, capData] = await a(capClient);
+    assert.equal(capEv, 'room_data');
+    assert.ok(Array.isArray(capData.capabilities) && capData.capabilities.includes('host-control'),
+        'ROOM_DATA advertises the host-control capability');
+    close();
+    resetConnectionRate();
+
     // --- Host Control Mode ---
     const hrid = 'host-'+Date.now();
     const h1 = await c(), h2 = await c();          // h1 = host (first joiner), h2 = guest