feat(brahman-broker-explorer): nueva app probe del broker brahman

Iter 14. Cierra otro frente: visibilidad del broker handshake. La
app probe cada 5s vía await_provider_blocking y reporta 4 estados
claros (Pending / Down / UpNoProvider / UpWithProvider).

crates/apps/brahman-broker-explorer/:
- Deps: brahman-handshake + brahman-sidecar + stack yahweh themed.
- ProbeState enum con 4 variants.
- Polling cx.spawn cada 5s; el probe blocking se ejecuta en
  cx.background_executor().spawn para no congelar el main thread.
- Configuración via env: BRAHMAN_INIT_SOCKET, BRAHMAN_BROKER_PROBE_FLOW
  (default broker-health), BRAHMAN_BROKER_PROBE_TYPE (default ping).
- UI: header probe info + theme switcher; banner permanente
  Error/Warning/Success según estado; stat card con accent color.
- 2 tests sanity.

Smoke run verificado: bootstrap OK, panic esperado sin display.

Apps GUI themed: 5 (nakui-ui + 3 explorers + ahora broker-explorer).

Limitaciones: observer Card se registra/desregistra en cada probe;
no muestra lista global de Cards (handshake no expone API). Para
timeline real de MatchEvents hace falta mantener el Client vivo
entre probes — scope futuro.

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

Cargo.lock

@@ -1235,6 +1235,19 @@ dependencies = [
  "ulid",
 ]
 
+[[package]]
+name = "brahman-broker-explorer"
+version = "0.1.0"
+dependencies = [
+ "brahman-handshake",
+ "brahman-sidecar",
+ "gpui",
+ "yahweh-theme",
+ "yahweh-widget-banner",
+ "yahweh-widget-card",
+ "yahweh-widget-theme-switcher",
+]
+
 [[package]]
 name = "brahman-card"
 version = "0.1.0"