feat: add tracker runtime and durable offline queue

[alirezat66]

Summary

Adds the production runtime responsible for tracker lifecycle, event delivery, failure isolation, offline persistence, and retry.

Changes

• Adds coroutine-native FlexTrackClient
• Adds tracker registration and lifecycle management
• Delivers events concurrently to routed trackers
• Isolates individual tracker failures
• Adds in-memory event queue
• Adds durable atomic-file event queue for Android
• Queues events when the device is offline
• Retries only tracker destinations that remain unsuccessful
• Preserves queued events across application restarts
• Handles coroutine cancellation correctly
• Executes the shared Flutter/Kotlin Core MVP fixtures in tests

Reliability

• One failing tracker does not prevent other trackers from receiving an event
• Queue writes are atomic to protect against interrupted application writes
• Successfully delivered destinations are not retried
• Consent-rejected events are neither delivered nor queued

Verification

• 28 Debug tests pass
• 28 Release tests pass
• All 8 shared conformance fixtures pass
• Android lint passes
• Release AAR generation passes
• Maven Local publication passes