Strict Type Coercion in DataContext

# Proposal: Strict Type Coercion in DataContext

## Reason
The A2UI protocol defines standard coercion rules (e.g., `null` resolving to `""` for strings). Without central enforcement, component authors must manually handle these edge cases, leading to bugs like `[object Object]` appearing in text labels.

## Design
1. **Coercion Utils**: Implement the coercion table from the `renderer_guide.md` in a shared utility.
2. **Context Integration**: Update `DataContext.resolveDynamicValue` and `resolveSignal` to apply these rules.
3. **Primitive Safety**:
- String targets: Convert any non-string to localized string; map `null/undefined` to `""`.
- Number targets: Parse strings; map `null/undefined` to `0`.
- Boolean targets: Map `"true"` (case-insensitive) and non-zero numbers to `true`.
4. **GenericBinder Alignment**: Ensure the `GenericBinder` uses these coerced values when populating `ResolvedProps`.

## Key Files
- `renderers/web_core/src/v0_9/rendering/data-context.ts`
- `renderers/web_core/src/v0_9/rendering/generic-binder.ts`