implement core behavior

[alirezat66]

Summary

Implements the core policy layer of FlexTrack Kotlin in alignment with the Flutter SDK and Core Specification 1.0.0.

Changes

• Adds immutable events with stable IDs and UTC timestamps
• Adds event enrichment and ordered transformer pipelines
• Adds consent-aware routing
• Adds priority-based routing rules and tracker groups
• Adds fallback routing and same-priority target merging
• Adds deterministic UTF-8 FNV-1a sampling
• Adds routing diagnostics and skipped-rule reasons
• Preserves event identity during enrichment
• Adds Android API 21 support for java.time

Compatibility

This is the initial Kotlin implementation, so there are no existing Kotlin users requiring migration.

Verification

• 15 Debug unit tests pass
• 15 Release unit tests pass
• Android lint passes
• Release AAR generation passes