feat(mix): add reverse animation config for style exit transitions

[tilucasoli]

Summary

Closes #929.

Adds an optional reverse parameter to animate so a variant style can own both its enter and exit transition configs, instead of splitting an interaction's enter/exit timing across two styles:

BoxStyler()
    .animate(baseAnimation)
    .onHovered(
      BoxStyler()
          .scale(1.02)
          .animate(hoverIn, reverse: hoverOut),
    );

When the hover style becomes active, Mix uses hoverIn; when it is no longer active, Mix uses hoverOut. With no reverse declared, behavior is unchanged.

Semantics

• Use the target style's forward config when entering it.
• Use the leaving style's reverse config when leaving it.
• Only use the old reverse config when the old and new animation metadata are distinct — so an inherited base reversible config is not mistaken for a hover exit animation.
• reverse is an exit transition config; it does not mean calling AnimationController.reverse().

Changes

Core (packages/mix)

• animation/animation_config.dart — new ReversibleAnimationConfig wrapping forward + reverse.
• animation/style_animation_builder.dart — selects the effective transition config from old/new metadata (_transitionConfig / _forwardOf / _reverseOf) and tracks the active driver's config kind, so forward and reverse may use different driver kinds without type errors.
• style/mixins/animation_style_mixin.dart — abstract animate gains {AnimationConfig? reverse}.

Generator (packages/mix_generator)

• Generated animate instance methods and static factories accept {AnimationConfig? reverse} and wrap in ReversibleAnimationConfig only when reverse is provided.
• Regenerated all 8 *_spec.g.dart files.

Acceptance criteria

• Existing .animate(config) calls continue to compile.
• Generated stylers support .animate(config, reverse: reverseConfig).
• Static factories support Styler.animate(config, reverse: reverseConfig).
• Hover enter uses the hover style's forward config.
• Hover exit uses the hover style's reverse config when present.
• Hover exit falls back to the target/base config when no reverse config is present.
• An inherited base reversible animation does not cause hover enter to use reverse.
• Forward and reverse configs may use different driver kinds without type errors.

Test plan

• mix: animation + hover suites green (test/src/animation/, test/src/core/style_builder_hover_test.dart); full suite 2756 passing.
• mix_generator: full suite (257) passing; updated signature assertions + added ReversibleAnimationConfig to the mock mix stub.
• New tests: ReversibleAnimationConfig equality/props, .animate(reverse:) wrapping (instance + static factory), and hover enter/exit + inherited-config widget tests.

🤖 Generated with Claude Code