Unused token reference types in prop_refs.dart and token_refs.dart

## Summary

After dropping `TextDecorationRef` in #c68d30953 (because `TextDecoration` became a `final class` in newer Flutter SDKs), it became clear that many other ref types in `packages/mix/lib/src/core/prop_refs.dart` and `packages/mix/lib/src/theme/tokens/token_refs.dart` are also dead code — they're declared, exported, and matched in `assertIsRealType`, but nothing in the codebase ever produces an instance.

We should decide what to do with them: either wire them up to real `MixToken` subclasses (so users can actually tokenize these types) or remove them.

## Unused refs

**In `prop_refs.dart`** (31 classes, never instantiated by `getReferenceValue` or any `MixToken`):

- `AlignmentGeometryRef`, `AlignmentRef`, `AlignmentDirectionalRef`
- `FontFeatureRef`
- `OffsetRef`, `RectRef`
- `LocaleRef`
- `ImageProviderRef`, `DecorationImageRef`
- `GradientTransformRef`, `GradientRef`, `LinearGradientRef`, `RadialGradientRef`, `SweepGradientRef`
- `Matrix4Ref`
- `TextScalerRef`, `StrutStyleRef`, `TextHeightBehaviorRef`
- `TableColumnWidthRef`, `TableBorderRef`
- `BoxBorderRef`
- `BorderRadiusGeometryRef`, `BorderRadiusRef`, `BorderRadiusDirectionalRef`
- `EdgeInsetsGeometryRef`, `EdgeInsetsRef`, `EdgeInsetsDirectionalRef`
- `BoxDecorationRef`
- `ShapeBorderRef`
- `BoxConstraintsRef`
- `CurveRef`

**In `token_refs.dart`** (4 classes, unreachable — actual shadow tokens are list-based):

- `ShadowRef`, `BoxShadowRef`
- `ShadowMixRef`, `BoxShadowMixRef`

## Why this matters

- They appear in the public API surface (exported) but can't be created by users, so they're misleading.
- They bloat `assertIsRealType` with branches that are never exercised in real flows.
- Some (like `TextDecorationRef`) actively break compilation when Flutter marks the implemented type `final` or `sealed`. Several of the above (e.g. `Alignment`, `EdgeInsets`, `BoxConstraints`) are also at risk.
- The token-migration guide currently advertises some of these as supported, which is stale.

## Options

1. **Remove them all** (and trim `assertIsRealType` + `getReferenceValue` + the migration guide).
2. **Add corresponding `MixToken` subclasses** so they become real, supported token types.
3. **Hybrid** — keep the ones we plan to support and remove the rest.

Worth a discussion before picking a direction.