F-103: hybrid targets progressive example (flat api/impl/stub)

[stepango]

Summary

• Adds examples/android/15-hybrid-targets teaching co-located feature/{hello,world}/{api,impl,stub-impl} (GH Create hybrid targets example #44).
• Stubs are real impl modules (packageName …stub); bare stub/ is invalid under the -impl suffix rule — documented.
• Composition roots default to api + impl; commented manual swap to stub-impl is an F-104 teaser only.
• Curriculum: progressive matrix row 15, examples ladder, forma-project-layout skill; F-103 → done.

Non-goals

• No F-104 IDE swap APIs
• No plugins/ / matrix engine changes
• No androidLibrary

Verify

source scripts/env-mac.sh
cd examples/android/15-hybrid-targets
printf 'sdk.dir=%s\n' "$ANDROID_HOME" > local.properties
./gradlew :binary:assembleDebug
# BUILD SUCCESSFUL
./gradlew :feature-hello-stub-impl:compileDebugKotlin :feature-world-stub-impl:compileDebugKotlin

Test plan

• :binary:assembleDebug green locally
• Stub modules compile
• Project names: :feature-hello-{api,impl,stub-impl}, :feature-world-{api,impl,stub-impl}
• Curriculum links present; no engine churn