feat(mix): rework Pressable input and semantics, add focus-visible variant

[leoafarias]

Summary

Interaction-subsystem changes to core mix extracted from the Tailwind parity branch (leoafarias/mix-tailwind-compat). They were found during parity work, but none of them are Tailwind-specific — they are standalone framework fixes plus two general-purpose features, so they ship as their own PR. The parity branch stacks on top of this one.

Bug fixes (real bugs, reproducible without mix_tailwinds)

• Nested widget-state discovery (style.dart): Style.widgetStates only scanned top-level WidgetStateVariants. A style whose only widget-state variants were nested (onDark(BoxStyler().onHovered(...)), onBreakpoint(...)) or negated (onNot(hover)) reported no dependencies, so StyleBuilder never attached a MixInteractionDetector and the variant never activated. Now recursive with identity-based cycle protection, and driven by the new ContextVariant.widgetStateDependencies so non-WidgetStateVariant variants (Not, FocusVisible) participate.
• Duplicate pressed-state owners (pressable_widget.dart): both GestureDetector (tapDown/tapUp/tapCancel) and MixInteractionDetector's Listener wrote pressed to the same controller, with conflicting timing (e.g. tapCancel during a long-press cleared pressed while the pointer was still down). The Listener is now the single owner.
• Controller lifecycle: late final _controller ignored widget.controller swaps, and dispose() consulted the current widget to decide ownership — swapping external↔internal could dispose a controller the widget didn't own or leak the one it did. Fixed via _ownsController + didUpdateWidget.
• Semantics correctness: the semantics node exposed onTap even when disabled, had no enabled state or longPress action, and GestureDetector's implicit semantics duplicated actions. Actions are now gated on enabled, enabled is exposed, and the GestureDetector is always excluded from semantics.
• PressableBox.enableFeedback existed but was never forwarded to Pressable; the full surface (cursor, focus, keyboard, controller, actions, semantics) is now forwarded.
• Keyboard activation previously bound ActivateIntent → onPress with no enabled check and no pressed feedback. Now Space/Enter/numpad Enter model held state: pressed on key down, activate once on key up, repeats suppressed, and cancellation on focus loss, disable, controller swap, and dispose.

New features

• FocusVisibleVariant / ContextVariant.focusVisible() / onFocusVisible(...): CSS :focus-visible equivalent, keyed off FocusManager.highlightMode (the same input-modality signal FocusableActionDetector uses), made reactive inside Pressable via the new internal FocusHighlightModeProvider.
• PressableSemanticsRole (button / link / none) + semanticsLabel.

mix_protocol's schema inventory registers the new public symbols as v1 out-of-scope.

Breaking changes

• semanticButtonLabel → semanticsLabel (no remaining usages in-repo).
• Deprecated onKey removed; use onKeyEvent (it runs first and can override activation).
• Pressable reserves Space/Enter/numpad Enter: custom actions no longer receive ActivateIntent for those keys. Deliberate contract so held-key state stays consistent; documented in the CHANGELOG.

Note one judgment call: activation fires on key-up for both Space and Enter (the web fires Enter on key-down). The uniform key-up model was chosen so the held/pressed lifecycle is consistent across keys.

Test plan

• packages/mix: full suite passes (2,852 tests), including new coverage: keyboard hold/activate-once/cancel paths, controller-swap state transfer, onKeyEvent precedence, reserved-keys-vs-custom-actions, focus-visible modality switching, semantics role/action matrix, and nested/negated/cyclic variant discovery.
• packages/mix_protocol: inventory + contract tests pass.
• dart analyze packages/mix packages/mix_protocol: no issues.

[leoafarias]

Superseded by the split into #1006 (nested widget-state discovery fix, independently landable) and #1007 (Pressable input/semantics rework + focus-visible, stacked on #1006). Same combined tree, reorganized for review.