F-013: Compose support for Android targets

[stepango]

Summary

Implements F-013 / GH #96 Compose support:

• compose flag on impl, androidLibrary, androidUtil, androidApp, uiLibrary, androidBinary
• Project composeCompilerVersion (default 1.5.3 for Kotlin 1.9.10)
• New composeWidget target (suffix compose-widget, always enables Compose)
• Validators allow compose-widget from UI/composition consumers; widget and compose-widget may depend on each other
• Sample: application/common/greeting/compose-widget + binary compose=true
• Sample compileSdk 34 for Compose AAR metadata; CI installs platform 34
• Docs: docs/COMPOSE.md, matrix/README/ARCHITECTURE/ENV

Verify

• plugins/: ./gradlew build -> BUILD SUCCESSFUL
• application/: ./gradlew build -> BUILD SUCCESSFUL (2151 tasks)

Next

F-014 sample app gold-standard structure