Replace the implicit "hostPeerId present ⇒ feature supported" heuristic with an
explicit capabilities list the relay advertises in ROOM_DATA. Cleaner, self-
documenting, and the extensible hook for the planned co-host feature (owner
promotes guests to extra controllers) — a future 'co-host' capability + events
slot in without a protocol bump.
- shared: CAPABILITIES { HOST_CONTROL }.
- server: SERVER_CAPABILITIES advertised in ROOM_DATA.
- background: track serverCapabilities (empty against older relay), serverSupports(),
thread hostControlSupported through GET_STATUS / GET_CONTROL_MODE / CONTROL_MODE.
- popup: gate the host-control card on the explicit capability instead of hostPeerId.
Backwards-compatible both ways: old relay omits the field → feature stays hidden
(no errors); old client ignores the field. WS test asserts ROOM_DATA advertises
the capability. Adds docs/host-control-mode-TESTING.md (beta-server setup, wss
caveat, two-new-clients note, verification checklist).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
KoalaDev