- All ARCHITECTURE §3 interfaces as pure types; dependency-free (no ws/node/DOM)
so both backend and frontend can import it
- Refinements vs §3 (documented): EnvLike (not NodeJS.ProcessEnv), WebSocketLike +
WS_OPEN (not ws.WebSocket), added Config.wsPath (invariant 8)
- ARCHITECTURE §3.1/§3.4 reconciled to match (anti-drift)
- tsc --noEmit passes
T2 of docs/PLAN.md
