feat: Add protocol spec, rate limiting, vitest framework, and documentation

## Added
- **WebSocket Protocol Specification** (docs/PROTOCOL.md) - Complete reference
  for all 20+ events with payload schemas, rate limits, and edge cases
- **LEAVE_ROOM Rate Limiting** - 10 requests/socket/minute to prevent abuse
- **Vitest Testing Framework** - Modern test setup with coverage reporting
- **Host Control Mode Documentation** - Consolidated EN documentation
  (docs/host-control-mode.md) with references to internal implementation

## Changed
- **Graceful Shutdown** - Socket.IO clients properly disconnected during
  server shutdown (server/index.js)
- **CI Workflow** - Added test step with continue-on-error for gradual rollout
- **README** - Added protocol documentation link
- **CHANGELOG** - Updated with all new features and improvements

## Moved
- Internal documentation moved to docs/internal/ for better organization
- Host Control Mode docs consolidated from 4 files to 1 comprehensive guide

## Technical Details
- Protocol spec: 495 lines, covers all events from shared/constants.js
- Rate limiter: Follows existing pattern (checkAuthRate, checkEventRate)
- Vitest: 6 tests for rate limiter, all passing
- Documentation: Host Control Mode doc includes edge cases, testing checklist,
  architecture decisions

.github/workflows/ci.yml

@@ -42,5 +42,9 @@ jobs:
         run: npm ci
         working-directory: server
 
-      - name: Run verification suite
-        run: npm run verify
+       - name: Run tests
+         run: npm test
+         continue-on-error: true
+
+       - name: Run verification suite
+         run: npm run verify