Apply Kotlin's binary-compatibility-validator at the root project so both
:sharingan and :sharingan-noop get apiDump/apiCheck. Commit the golden public
-API dumps (Android/JVM .api and iOS .klib.api) for each module and run
apiCheck on every PR via a new macOS workflow (macOS so the iOS klib ABI
targets build alongside the Android/JVM surface).

The "swap sharingan-noop in release" safety story depends on the two artifacts
keeping an identical, stable public API; nothing enforced that before. apiCheck
now fails the build when the surface drifts from the committed dumps. The sample
app is excluded (not a published library). Documented apiDump/apiCheck in
AGENTS.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>