Simplify internals and move performance stress test to benchmark

[KickNext]

What changed

• deduplicate package-version loading in the example
• replace private data holders with records and manual list comparisons with listEquals
• remove the PERF showcase tab and its widget-test surface
• add a standalone macOS profile integration benchmark covering plain/chromatic workloads at 24, 96, and 192 concurrent ReelText widgets
• document the benchmark command

Why

The showcase carried a large performance-only UI surface and duplicated small pieces of internal machinery. Keeping stress profiling in an integration benchmark reduces production/example surface area while preserving repeatable performance measurements.

Performance profile

Two macOS profile runs completed successfully. Raster stayed below budget with zero missed raster frames. The measured bottleneck is UI-thread work, especially per-frame chromatic text layout and transition-start text measurement; renderer optimizations are intentionally deferred to a follow-up branch.

Validation

• flutter analyze
• flutter test (101 tests)
• cd example && flutter analyze
• cd example && flutter test (23 tests)
• two macOS profile runs of performance_benchmark_test.dart
• dart pub publish --dry-run (0 warnings)