feat(yahweh-widget-banner): widget compartido para toasts/errores cross-app

Patrón visual común a yahweh-widget-meta-form (toast success +
error banner) y nakui-explorer (error banner): div con bg + text
colored según severidad. Antes duplicado con colores hardcoded en
cada consumer.

Crate nuevo crates/modules/ui_engine/widgets/banner:
- pub enum Banner { Info, Success, Warning, Error } con bg()/fg()
  hardcoded por variant.
- pub fn banner(kind, message) -> Div: padding/text_size defaults;
  caller compone con .child()/.px()/.on_click()/etc.
- 2 tests sanity (no color collisions).

Migración:
- yahweh-widget-meta-form: 12 líneas hardcoded → 2 llamadas a
  banner().
- nakui-explorer: error banner usa banner() + override de
  padding custom del header.

Tests stack: 109 → 111 (+2). Cada crate compila individualmente.

Próximo natural: confirm_delete_banner (Warning + botones) puede
extraerse como modal-banner cuando emerja segundo consumer.

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

Cargo.lock

@@ -6460,6 +6460,7 @@ dependencies = [
  "tempfile",
  "uuid",
  "yahweh-meta-runtime",
+ "yahweh-widget-banner",
 ]
 
 [[package]]
@@ -12998,6 +12999,13 @@ dependencies = [
  "gpui",
 ]
 
+[[package]]
+name = "yahweh-widget-banner"
+version = "0.1.0"
+dependencies = [
+ "gpui",
+]
+
 [[package]]
 name = "yahweh-widget-container-core"
 version = "0.1.0"
@@ -13016,6 +13024,7 @@ dependencies = [
  "yahweh-meta-runtime",
  "yahweh-meta-schema",
  "yahweh-theme",
+ "yahweh-widget-banner",
  "yahweh-widget-text-input",
 ]