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>

AGENTS.md

@@ -15,9 +15,7 @@ AGENTS.md. Source of truth: the code, then the docs below.
 - Each fact lives in exactly one file; others link to it.
 - Code and its docs change in the same commit. A wire change also bumps
   `RelayConstants.protoVersion` and edits PROTOCOL.md.
-- User-facing wording says "server" (there is only one), never "room".
-  Code identifiers and wire names keep their real names until the rename
-  in ROADMAP item 1.
+- Say "server" (there is only one), never "room", in UI, code and docs.
 
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