Optimize WaveWidget painting for long durations

[RockerFlower]

Summary

• bump package version to 0.2.5 for the issue Long durations make animations stutter #53 performance fix
• reduce per-frame wave path work by sampling the curve instead of adding one point per pixel
• reuse painter path and gradient shader state, and isolate WaveWidget with a repaint boundary
• avoid recreating animation controllers when runtime-only paint parameters such as amplitude change
• add issue Long durations make animations stutter #53 regression coverage and a manual benchmark test

Fixes #53

Benchmark

Scenario: issue #53 reproduction config, Flutter 3.7.12 via FVM, 1440x300 WaveWidget, four gradient layers, durations 43000/43000/45000/45000, waveAmplitude 180, waveFrequency 1, 1800 pumped frames.

Command:

fvm flutter test tool/issue_53_benchmark_test.dart --reporter expanded

Revision	Average pump time
master / 0.2.4 baseline	0.258 ms/frame
this branch / 0.2.5	0.128 ms/frame

Result: about 50% lower frame pump time, roughly 2.0x faster in this benchmark.

Verification

• fvm flutter analyze
• fvm flutter test
• fvm dart pub publish --dry-run: 0 warnings
• fvm flutter test tool/issue_53_benchmark_test.dart --reporter expanded