New Features:
- Add callAfterFirstBuild() for executing callbacks after first frame render
- Add callAfterEveryBuild() with cancel callback for per-rebuild operations
- Add const_widget_test.dart to verify const widgets work with WatchItMixin

Breaking Changes:
- Migrate from functional_listener to listen_it dependency
- Change watchValue() and registerHandler() to default allowObservableChange: false
  for better performance and memory leak prevention

Performance Improvements:
- Selectors now only called once on first build (zero overhead on rebuilds)
- Add helpful StateError messages for unexpected observable changes

Bug Fixes:
- Fix createOnceAsync test isolation issue

Tests: All 57 tests passing