feat!: replace Flutter bridge with Dart OpenFeature provider

[nicklasl]

Replace the native Flutter bridge with confidence_openfeature_provider at the repository root. The provider resolves flags in Dart, supports synchronous typed OpenFeature reads and context reconciliation, and reports exposures only after successful reads.

Migrate native visitor identity, cached flags and queued telemetry with durable import checkpoints. An atomic JSON outbox provides batching, retries and public flush; its 4 MiB budget drops the oldest unsent records. The documented trade-offs include asynchronous persistence and possible duplicate delivery after a crash. A small Foundation helper preserves legacy Swift calendar values without a native Confidence SDK dependency.

Update the public-API example, credential-free CI, migration guide and release configuration. Publishing remains disabled with publish_to: none.

Validation:

• 157 offline tests, static analysis and formatting pass.
• Live resolution, exposure delivery and tutorial-event acceptance pass for both Flutter SDK IDs.
• Retained-sandbox upgrades pass on Android API 36 and iOS 26.3, including offline import, replay after restart and no repeat replay. These use native fixtures and simulated HTTP.
• Android and iOS integration tests and Android, iOS SPM and CocoaPods builds pass.
• Publication dry run passes with five documented warnings for deliberate dependency pins. No package has been published; broader OS coverage and publisher ownership remain first-release checks.

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