build: establish Kotlin library foundation and shared contract

[alirezat66]

Summary

Establishes the initial FlexTrack Kotlin project structure and adopts the shared FlexTrack Core MVP contract.

Changes

• Adds the publishable flextrack Android library module
• Converts the application module into the sample app
• Targets Android API 21+
• Configures AAR and Maven Local publication
• Adds Maven POM metadata
• Adds CI checks for build, tests, lint, and publication
• Vendors Core Specification 1.0.0 and shared conformance fixtures
• Adds architecture, changelog, license, and initial documentation

Verification

• Library build passes
• Android lint passes
• Release AAR generation passes
• Maven Local publication passes