feat: implement Tier 3 features — lane queue, credential redaction, token dashboard, xAI, Voyage AI

- Lane Queue: per-session FIFO queue in gateway replacing reject-when-busy (9 tests)
- Credential Redaction: redactConfig() expanded to cover 18+ secret fields (16 tests)
- Web UI Token Dashboard: system.tokenUsage endpoint + Usage page with summary cards
- xAI (Grok) Provider: OpenAI-compatible client with model pricing
- Voyage AI Embeddings: new embedding provider with configurable dimensions (5 tests)
- Update gap analysis: 90→95 match (70%→74%), Tier 3 section marked DONE
- Update state.json: test count 1001→1034, add tier3_completion entry

Total: 1034 tests passing across 85 files, typecheck clean

src/gateway/index.ts

@@ -4,6 +4,7 @@ export { Router } from './router.js';
 export type { HandlerFn, SendFn } from './router.js';
 export { SessionBridge } from './session-bridge.js';
 export type { SessionBridgeConfig } from './session-bridge.js';
+export { LaneQueue } from './lane-queue.js';
 export { authenticateRequest } from './auth.js';
 export type { AuthConfig, AuthResult } from './auth.js';
 export { serveStatic } from './static.js';