feat(protocol): add host control mode events & constants

Step 1 of host control mode. Adds the protocol surface only — no behavior yet:
- EVENTS.SET_CONTROL_MODE (client->server: host sets mode)
- EVENTS.CONTROL_MODE   (server->client: mode changed)
- CONTROL_MODES { EVERYONE, HOST_ONLY }

Purely additive; old clients ignore the new events. Propagated to
extension/shared via build-extension. Also records the audited design
decisions in the edge-case log.

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

shared/constants.js

@@ -33,6 +33,10 @@ export const EVENTS = {
     ROOM_DATA: "room_data", // Server -> Client: current room state
     ERROR: "error",
 
+    // Host Control Mode
+    SET_CONTROL_MODE: "set_control_mode", // Client -> Server: host sets room control mode ('everyone' | 'host-only')
+    CONTROL_MODE: "control_mode",          // Server -> Client: room control mode changed { controlMode, hostPeerId }
+
     // Media Control
     PLAY: "play",
     PAUSE: "pause",
@@ -57,6 +61,14 @@ export const EVENTS = {
     PONG: "pong"   // server responds with same { t } for client RTT calculation
 };
 
+// Room control modes (Host Control Mode feature).
+// NOTE: content.js does not import this module — it uses the string literals
+// 'everyone' / 'host-only' directly. Keep these values in sync there.
+export const CONTROL_MODES = {
+    EVERYONE: 'everyone',   // default: anyone can play/pause/seek for the room
+    HOST_ONLY: 'host-only'  // only the host drives the room
+};
+
 export const HEARTBEAT_INTERVAL = 15000; // 15s
 export const FORCE_SYNC_TIMEOUT = 8500; // 8.5s timeout for force sync ACKs (must be > content.js poll timeout of 8s)
 export const EPISODE_LOBBY_TIMEOUT = 60000; // 60s timeout for episode lobby