feat: add initialPhase and onPhaseUpdate parameters

[FinSchuiling]

Summary

Adds two optional parameters to AnimatedMeshGradient:

• initialPhase (double?): When non-null, the animation starts from this phase value instead of 0. Useful for resuming from a previously saved state (e.g. restoring the gradient to exactly where it was when the user navigated away). Ignored if seed is set.

• onPhaseUpdate (void Function(double)?): Called each tick with the current animation phase. Allows consumers to persist the phase value so they can later restore it via initialPhase, or display a static gradient at the paused position using seed.

Example usage

AnimatedMeshGradient(
  colors: myColors,
  options: myOptions,
  controller: _controller,
  initialPhase: _savedPhase,       // resume from saved position
  onPhaseUpdate: (phase) {
    _savedPhase = phase;           // track current position
  },
)

Note: This PR is based on fix/listener-and-ticker-restart (#34) which fixes listener cleanup and ticker restart bugs.

Test plan

• Verify initialPhase: 5.0 starts the gradient visually offset (not from 0)
• Verify onPhaseUpdate fires each tick with increasing phase values
• Verify both parameters are ignored when seed is set
• Verify passing neither parameter preserves existing behaviour (starts from 0, no callback)

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