feat(brahman-handshake): ListMatches endpoint + timeline en broker-explorer

Iter 21. Cierra el loop iniciado en iter 20: ahora se ven sesiones
+ matches actuales + cómo cambian a través del tiempo.

brahman-handshake/messages:
- Frame::ListMatches → Frame::MatchList(Vec<brahman_broker::Match>).

brahman-handshake/server:
- run_post_handshake pasa Option<&SharedBroker> a handle_inbound_frame.
- Sin broker configurado → MatchList vacía (no error).

brahman-handshake/client + brahman-sidecar:
- Client::list_matches() análogo a list_sessions, drena MatchEvents.
- list_matches / list_matches_blocking, mismo patrón.

brahman-broker-explorer:
- Poll-tick agrega list_matches_blocking además de list_sessions.
- last_match_keys: HashSet<MatchKey> para diff entre ticks.
- timeline: VecDeque<TimelineEntry> cap 50.
- diff_matches (free fn): Available para keys nuevas, Lost para
  desaparecidas. Primer tick marca todo Available (boot UX).
- Render: stat_card "Timeline" con HH:MM:SS {+/-} formato compacto.

5 tests broker-explorer (3 nuevos del diff). Stack verde.

Decisión: timeline polled cada POLL_INTERVAL=5s, no push. MatchEvents
del broker son consumer-céntricos (cada session ve sólo SUS matches);
"system-wide timeline" requeriría broker subscribe-all (mucho scope).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -1239,9 +1239,12 @@ dependencies = [
 name = "brahman-broker-explorer"
 version = "0.1.0"
 dependencies = [
+ "brahman-broker",
+ "brahman-card",
  "brahman-handshake",
  "brahman-sidecar",
  "gpui",
+ "ulid",
  "yahweh-launcher",
  "yahweh-theme",
  "yahweh-widget-app-header",