v0.3.1 of firestore_access_policy

Declarative Firestore access policies in Dart — generate security rules, tests, and optional client guards from a single CRUD + membership model. Changelog excerpt: - De-brand docs; clarify that [FirestoreRulesFile.headerComment] is consumer-controlled. - [RulesFileDefaults] for neutral generated-rules headers. - Rename example to firestore_access_policy_example.dart(pub.dev recognized). - Dartdoc on public API; pub score fixes.