Server on an isolate, dedicated server binary, room -> server (proto 5)

- serve(Map config, events) is the one server entrypoint: the app
  hosts it via Isolate.spawn (crash surfaced on the Host card, logs
  forwarded as [server]); bin/server.dart is the dedicated server,
  compiled into the bundle as watch-together-server. --headless gone.
- Host identity via a per-hosting hostKey in hello, replacing the
  loopback heuristic.
- Rename room -> server across code, files and wire (t:"state",
  /health name); protocol bumped to 5.
- CI compiles the server binary; isolate smoke tests in server and
  desktop suites. Docs updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/README.md

@@ -1,5 +1,6 @@
 # apps/desktop
 
-The Flutter desktop app (the only binary users run). Layout and rules:
+The Flutter desktop app (what users run; hosting runs the server from
+`packages/server` on an isolate). Layout and rules:
 [../../docs/ARCHITECTURE.md](../../docs/ARCHITECTURE.md). Build and test:
 [../../docs/DEVELOPMENT.md](../../docs/DEVELOPMENT.md).