Allow Dart 3 package consumers

[RockerFlower]

Summary:

• Expand the package and example SDK constraints to allow Dart 3 while preserving the Dart 2.12 lower bound.
• Move master development to 0.2.4-dev.1 after the 0.2.3 release.
• Run CI against both Flutter 3.7.12 and the latest stable Flutter.

Validation:

• fvm flutter analyze
• fvm flutter test
• fvm flutter pub publish --dry-run (0 warnings)

Notes:

• The Cloudflare demo deploy remains pinned to Flutter 3.7.12 for now; this PR only changes package compatibility coverage.