feat: compile-check opt-in snippets and release 2.4.0

[aldefy]

Summary

• add opt-in kotlin compile blocks alongside existing kotlin verify layout assertions
• scan all top-level reference docs for executable markdown blocks
• emit compile-only generated subjects so stale APIs fail Kotlin compilation
• add one stable MaterialTheme compile-checked snippet
• bump plugin version to 2.4.0 and document the release

Verification

• ./gradlew -p buildSrc test
• ./gradlew :verify-claims:testDebugUnitTest --no-daemon
• bash scripts/check-versions.sh v2.4.0
• bash scripts/check-description-length.sh
• bash scripts/validate-plugin-manifest.sh
• bats scripts/check-versions.bats scripts/validate-plugin-manifest.bats

Negative probe: a temporary kotlin compile block calling DefinitelyNotAComposeApi() failed at compileDebugUnitTestKotlin with an unresolved reference, then was removed and the suite returned green.