Fix animation transition lifecycle and target correctness

[leoafarias]

Summary

Repair curve, spring, phase, and keyframe driver transitions so interrupted
animations hold the visible value, removing a configuration reaches the new
target, phase delays use the correct transition index, and completion
listeners fire exactly once. Treat this as one lifecycle-contract change —
fixing only one symptom risks leaving another old/new driver combination
inconsistent.

Why this matters

Several behaviors can render something other than the requested target:

1. CurveAnimationDriver builds its delay with ConstantTween(_initialSpec).
   When an in-progress transition is interrupted, the next delay snaps back to
   the driver's first target instead of holding the currently visible
   interpolated spec.
2. StyleAnimationBuilder replaces a driver with config ?? oldConfig on type
   change. That fallback is valid for implicit curve/spring drivers (they
   implement didUpdateSpec) but phase/keyframe drivers do not retarget there.
3. Removing a looping phase/keyframe config can therefore create a new driver
   from the old config and keep the old sequence running instead of displaying
   the new unanimated target.
4. Phase tween construction checks configs[currentIndex].delay but weights the
   inserted delay with configs[nextIndex].delay, so placement and duration
   disagree.
5. _setUpAnimation adds anonymous phase status listeners on every update and
   never removes them, so completion callbacks accumulate.
6. PhaseAnimationConfig.onEnd exists but the driver reads the last curve
   config's callback instead.

Source anchors

• packages/mix/lib/src/animation/style_animation_builder.dart
• packages/mix/lib/src/animation/style_animation_driver.dart
• packages/mix/lib/src/animation/animation_config.dart

Required behavior

• Interrupted implicit animations begin from the currently displayed
  StyleSpec; a configured delay holds that value for its full duration with no
  backward jump.
• Reuse an outgoing config for a null target only when the driver supports
  retargeting (curve, spring). Removing phase/keyframe config must stop/dispose
  the old driver and expose the supplied target on the same update.
• The full old→new driver matrix (null, curve, spring, phase, keyframe) must
  dispose old listeners/tickers and reach the correct target.
• Each phase transition uses one consistent destination index for its delay,
  duration, curve, and callback.
• Choose one phase completion contract and fire it exactly once; do not retain
  two competing onEnd paths.
• Fail early with actionable errors for empty phase lists, unequal style/
  curve-config counts, negative durations/delays, looping timelines with no
  positive duration, and duplicate keyframe track IDs.

Acceptance criteria

• Interrupted delayed transitions hold the visible value without snapping.
• Removing phase/keyframe animation reaches the new target immediately and stops old tickers.
• Curve/spring outgoing transitions retain their animate-out behavior.
• Phase delay placement matches configured durations and delays exactly.
• Completion and trigger listeners never accumulate across config updates.
• Invalid phase/keyframe configurations fail before controller execution.
• The full driver-transition matrix is represented by tests.
• New regression tests fail on main and pass with the fix.