[rwrife]

Summary

• Add pure Dart event, volunteer, shift and assignment domain with coverage and UTC-based overlap checks.
• Add SQLite schema v2, transactional migration and atomic whole-plan save/load. Preserve attendance and private notes locally.
• Validate DST offsets and millisecond boundaries, with tests for migration, reload, rollback, conflicts, duplicate assignment, zero/under/overfill, midnight and DST fall-back.

Verification

• Flutter 3.47.7 / Dart 3.13.5: flutter pub get --enforce-lockfile
• dart format --output=none --set-exit-if-changed lib test
• flutter analyze — No issues found
• flutter test — 10 passed on Linux host
• Independent staged-diff review — no material correctness issues after precision fix
• Windows and Android builds: CI pending

Scope: shell remains unconnected to domain/store; issue #3 will wire authoring to persistence. iOS compile not available on Linux.

Closes #2