feat(yahweh-widget-card): container card-shape compartido para timeline entries

Iter 3 de integración nakui↔yahweh. El card visual pattern (padding
consistente + rounded + flex_col + gap) que vivía duplicado en cada
timeline entry de nakui-explorer ahora es un widget yahweh reusable.

crates/modules/ui_engine/widgets/card/:
- pub fn card() -> Div: flex_col + px(12) + py(8) + mb(4) +
  rounded(4) + gap(2). Sin colores (caller decide via builder).
- 1 test smoke.

nakui-explorer:
- Los 2 timeline entry patterns (Seed/Morphism) pasan de ~7
  calls a ~3, intención "card with accent" emerge del nombre.

Tests stack: 111 → 112. App-agnostic — el widget no impone paleta,
permite themes diversos.

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

Cargo.toml

@@ -64,6 +64,7 @@ members = [
     "crates/modules/ui_engine/widgets/text_input",
     "crates/modules/ui_engine/widgets/meta-form",
     "crates/modules/ui_engine/widgets/banner",
+    "crates/modules/ui_engine/widgets/card",
 
     # ============================================================
     # modules/nakui/ — ERP matemático (nakui absorbido)