fix(mix): accept shadow token .mix() on raw-list styler methods (sentinel-style, non-breaking)

[tilucasoli]

Summary

Same goal as #932 — let BoxShadowToken('x').mix() / ShadowToken('x').mix() be passed directly to the shadow styler methods (BoxStyler.boxShadows, BoxStyler.shadows, FlexBoxStyler.shadows, StackBoxStyler.shadows, TextStyler.shadows) — but solved without changing any method signatures, so it is not a breaking change.

This is the sentinel-value approach, modeled on how DoubleRef lets a MixToken<double> flow through plain double APIs: the public type stays natural, the token ref is-a that type, and it's detected and routed at the call site.

Fixes #925.

The problem (recap of #925)

final token = BoxShadowToken('effect');
BoxStyler().boxShadows(token.mix());
// The argument type 'BoxShadowListMixRef' can't be assigned to
// the parameter type 'List<BoxShadowMix>'.

token.mix() returns a BoxShadowListMixRef (implements BoxShadowListMix), but the styler method takes a raw List<BoxShadowMix>.

Approach: sentinel-style refs (like DoubleRef)

DoubleRef is a sentinel-backed shim that implements double, so a MixToken<double> flows through any double parameter and is detected during prop construction. The signature never changes from double.

This PR applies the same idea to shadow lists:

1. BoxShadowListMixRef now also implements List<BoxShadowMix> (and ShadowListMixRef implements List<ShadowMix>). The ref already extends Prop<List<BoxShadow>> and mixes in ValueRef (whose noSuchMethod guards the list members, which are never actually invoked). So token.mix() now satisfies the existing List<BoxShadowMix> parameter type.
2. The styler methods keep their List<BoxShadowMix> / List<ShadowMix> signatures. They detect a token ref (it's also a BoxShadowListMix / ShadowListMix) and route it through Prop.mix(...) so the TokenSource is preserved; literal lists take the original path unchanged.

What changed

File	Change
packages/mix/lib/src/theme/tokens/token_refs.dart	BoxShadowListMixRef / ShadowListMixRef now also implements List<BoxShadowMix> / List<ShadowMix>.
packages/mix/lib/src/style/mixins/shadow_style_mixin.dart	boxShadows detects a BoxShadowListMix token ref and routes it as a Mix; literal path unchanged. Signature unchanged.
packages/mix/lib/src/style/mixins/decoration_style_mixin.dart	Same for shadows. Signature unchanged.
packages/mix/lib/src/style/mixins/text_style_mixin.dart	Same for text shadows. Signature unchanged.
packages/mix/test/.../shadow_list_token_integration_test.dart	New tests for .mix() through each styler, literal-list still works, and MixScope end-to-end resolution.
packages/mix/CHANGELOG.md	Fix entry under the existing token-reference "Unreleased" section.

Difference from #932

	#932 (wrapper type)	This PR (sentinel-style)
Signature	boxShadows(BoxShadowListMix)	boxShadows(List<BoxShadowMix>) — unchanged
Breaking?	Yes — literal callers must wrap with BoxShadowListMix([...])	No
Generated .g.dart factories	regenerated	untouched
mix_tailwinds parser	updated to wrap	untouched
Existing styler tests	updated to wrap literals	unchanged

// Works (token — the case from #925)
const cardShadow = BoxShadowToken('shadows.card');
BoxStyler().boxShadows(cardShadow.mix());

// Still works exactly as before (no migration)
BoxStyler().boxShadows([
  BoxShadowMix(color: Colors.black, blurRadius: 8),
]);

Test plan

• dart analyze lib clean (mix).
• dart analyze lib clean (mix_tailwinds) — confirms nothing downstream needed changing.
• flutter test mix specs/style/theme suites: 545/545 passing.
• flutter test mix_tailwinds: 322/322 passing.
• New integration tests (17 in the shadow token file) cover: .mix() implements the raw list types, each styler accepts .mix(), literal lists still compile/work, and MixScope resolves boxShadowToken.mix() / shadowToken.mix() to the configured values at render time.

DCM was skipped locally (not installed in this environment), same as #932. Should be re-checked by CI.

🤖 Generated with Claude Code

[leoafarias]

Review pass focused on the shadow list token behavior.

[leoafarias]

This branch currently blocks CI: with dcm analyze --fatal-warnings, boxShadow is statically List<BoxShadowMix>?, so DCM reports boxShadow is BoxShadowListMix and the following cast as unrelated. The same pattern in TextStyleMix fails too. Please route this through a DCM-safe helper or another preservation path so the token case survives without tripping analysis.

[leoafarias]

Since this ref now implements the raw List<ShadowMix> type, it can be passed to every existing List<ShadowMix> constructor, not just TextStyler.shadows. Generated IconStyler(shadows:) and IconThemeModifierMix(shadows:) still do Prop.mix(ShadowListMix(shadows)), which wraps this token ref as a list item and then tries to iterate it during resolve. Please apply the same token-preserving path to those raw-list shadow constructors, including the generated source for IconStyler.

[leoafarias]

This preserves pure token .mix() values, but merge resolution still loses token fields when a token list is chained with a literal list. In Prop.resolveProp, once sources include both TokenSource and MixSource, the resolved List<Shadow> must be converted back to Mix<List<Shadow>>; only individual Shadow/BoxShadow converters are registered today, so the token list is skipped. Please add list converters and tests for literal-to-token and token-to-literal merge order.