fix(mix): apply variants when resolving nested styles

[tilucasoli]

Problem

A Style stored inside another Style's Prop — i.e. a component sub-style — silently drops its own variants.

Prop.resolveProp() merges the MixSource values and then resolves them with mergedMix.resolve(context). For a Style, resolve() only resolves props; it does not apply variants. Context variants (widget states hovered/pressed/disabled/focused, brightness, breakpoints, platform, …) are applied solely by Style.build() (mergeActiveVariants(...) → resolve(...)). The top-level style goes through StyleBuilder → style.build(context), but every nested style prop went through resolve() — so its variants were quietly ignored.

This is what makes btwld/remix#59 fail:

RemixSelectStyle().trigger(
  RemixSelectTriggerStyle()
    .onHovered(RemixSelectTriggerStyle().labelColor(Colors.red)) // never applied
)

The trigger style is stored as a nested Prop<StyleSpec<RemixSelectTriggerSpec>>, so .onHovered / .onDisabled had no effect. (Dropdown items worked only because each is built through its own StyleBuilder + controller.)

Fix

Build nested styles instead of resolving them, so their context variants resolve against the current context:

resolvedValue = mergedMix is Style
    ? (mergedMix as Style).build(context) as V
    : mergedMix.resolve(context);

• No-op for nested styles without variants — mergeActiveVariants returns the style unchanged, then resolves identically to before. The blast radius is exactly "nested styles that carry variants."
• Only Style has build(); every other Mix (e.g. ModifierMix, DecorationMix, ShapeBorderMix) stays on the original resolve() path via the type guard.
• Type-safe: when mergedMix is a Style<S>, the prop's V is StyleSpec<S>, which is exactly what build() returns.

Verification

• ✅ Full mix test suite: 2716/2716 pass with the change.
• ✅ New regression test test/src/core/nested_style_variant_test.dart is red/green-proven (fails without the fix, passes with it).
• ✅ Downstream remix full suite: 1742/1742 pass — issue Pressable freezes on press when onPressed: null #59 is fixed with no remix-side code change.

Known boundary (follow-up, not addressed here)

StyleBuilder decides whether to auto-install a WidgetStateProvider from the top-level style's widgetStates only (it doesn't recurse into nested props). So a nested-only widget-state variant activates only when a state scope already exists (e.g. a component that passes a controller, as remix's Select does). Making Style.widgetStates recurse into nested style props is a larger, separate change worth a dedicated issue.

Fixes btwld/remix#59

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