security: harden server relay + documentation audit

Server Security (S-1 through S-8):
- S-1: Type-check and clamp peerId, protocolVersion, password
- S-2: Validate numeric/boolean/enum fields in relay peerData
- S-3: Construct explicit relay payload (stop spreading raw data)
- S-4: Type-check targetId and actionTimestamp in EVENT_ACK
- S-5: Restrict room IDs to [a-zA-Z0-9-] only
- S-7: Add eventCounts periodic cleanup alongside connectionCounts
- S-8: Guard version parsing against NaN bypass

Documentation (P-1, R-1 through R-6):
- P-1: Fix PRIVACY.md typo, document all in-memory data maps
- R-1/R-5: Fix stale sync-constants.bat references in shared/
- R-2: Fix stale lastTargetState ref in ARCHITECTURE.md
- R-3: Extension README title reflects cross-browser support
- R-6: Document content injection markers in scripts/README.md

docs/ARCHITECTURE.md

@@ -16,7 +16,7 @@ This document describes the communication flows and internal logic of the KoalaS
 ## 2. Media Event Synchronization
 When a user interacts with a video:
 1. **Detection**: `content.js` listens to native events (`play`, `pause`, `seeked`) on the `<video>` element.
-2. **Prevention of Loops**: Uses `lastTargetState` to distinguish between user actions and programmatic actions triggered by the extension.
+2. **Prevention of Loops**: Uses an `expectedEvents` Set to distinguish between user actions and programmatic actions. Expected events are consumed on match and expire via timeout.
 3. **Reporting**: `content.js` sends a `CONTENT_EVENT` to `background.js`.
 4. **Relay**: The Server forwards the event to all other peers in the room.
 5. **Execution**: Remote peers receive the command and call `video.play()`, `video.pause()`, or `video.currentTime = targetTime`.