feat(yahweh-widget-stat-card): promover patrón stat card como widget

Iter 15. El patrón "tarjeta de dashboard con border-l accent +
label + value grande + descripción + listing opcional" tenía 2
consumers (minga-explorer + brahman-broker-explorer). Ahora vale
extraer al stack yahweh.

crates/modules/ui_engine/widgets/stat-card/:
- pub fn stat_card(cx, label, value: impl Into<SharedString>,
  description, accent, text, text_dim, recent_items: &[String])
  -> impl IntoElement.
- Compone yahweh-widget-card::card_themed; sin theme directo
  (caller pasa text/text_dim ya resueltos).
- 3 tests #[gpui::test] con TestAppContext + theme: smoke con/sin
  items, type-check value.

minga-explorer:
- Borra fn stat_card local (~60 líneas).
- Borra dep yahweh-widget-card.
- 3 callsites pasan value.to_string() (widget acepta
  Into<SharedString>).

brahman-broker-explorer:
- fn state_card refactorizada como wrap del stat_card compartido,
  preservando la traducción ProbeState→(accent,value,description)
  como helper local app-specific.
- Borra dep yahweh-widget-card.

Sub-header del listing: pasa de "recent (N de TOTAL):" a
"recent (N):" — el widget no conoce TOTAL; el caller lo pone en
label si quiere ("Nodos AST (5 de 247)"). Trade-off aceptable
por reusabilidad genérica.

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

crates/apps/brahman-broker-explorer/Cargo.toml

@@ -10,7 +10,7 @@ brahman-handshake = { path = "../../core/brahman-handshake" }
 brahman-sidecar = { path = "../../shared/brahman-sidecar" }
 yahweh-theme = { path = "../../modules/ui_engine/libs/theme" }
 yahweh-widget-banner = { path = "../../modules/ui_engine/widgets/banner" }
-yahweh-widget-card = { path = "../../modules/ui_engine/widgets/card" }
+yahweh-widget-stat-card = { path = "../../modules/ui_engine/widgets/stat-card" }
 yahweh-widget-theme-switcher = { path = "../../modules/ui_engine/widgets/theme-switcher" }
 gpui = { workspace = true }