fix: add native submodule onboarding preflight

[papi-ux]

Summary

• add a Gradle preflight task that fails early when moonlight-common-c submodule files are missing
• wire that preflight before AGP external native build tasks so missing sources do not cascade into opaque ndk-build errors
• document clone/submodule recovery, pinned NDK version, and keep prebuilt native artifacts/AAR packaging as a separate release-engineering decision
• add a Python regression/source test and wire it into the helper/onboarding CI step

Closes #57

Validation

• python3 -m unittest tools.test_native_submodule_preflight — pass on MacPapi
• python3 -m unittest tools.test_nova_retroid_smoke tools.test_native_submodule_preflight — pass on MacPapi
• pc-papi clean worktree smoke before submodule init: ./gradlew :app:verifyMoonlightCommonCSubmodule --stacktrace fails with the new actionable submodule message
• pc-papi after git submodule update --init --recursive: ./gradlew :app:verifyMoonlightCommonCSubmodule --stacktrace — pass
• pc-papi: ./gradlew -PnovaAbis=x86_64 -PlintFailOnError=true lintNonRoot_gameDebug testNonRoot_gameDebugUnitTest --stacktrace — pass
• pc-papi: ./gradlew -PnovaAbis=x86_64 :app:assembleNonRoot_gameDebug --stacktrace — pass

Caveats

• MacPapi local Gradle remains blocked by missing Android SDK/local.properties, so Android Gradle verification ran on pc-papi.
• pc-papi ARM64 debug assemble currently hits an NDK 27 clang frontend crash in moonlight-common-c/src/Misc.c; x86_64 assemble and the submodule preflight pass, so this PR does not hide that separate native toolchain issue.