CPython 3.15.0 final: default pythonVersion, PBS 20261013, pinned checksums #196

CPython 3.15.0 is out on python.org. python-build-standalone 20261013 has 3.15.0 in both builds, GIL and freethreaded. python.org publishes python-3.15.0-{aarch64,x86_64}-linux-android.tar.gz and python-3.15.0-iOS-XCframework.tar.gz.

Scope:

1. gradle.properties: pythonVersion 3.14.7 → 3.15.0, pythonBuildStandaloneRelease 20260807 → 20261013. The fallback defaults in python-multiplatform/build.gradle.kts and python-multiplatform-gradle-plugin/build.gradle.kts follow.
2. python-checksums.properties:
   • add 3.15.0 for android aarch64 and x86_64, ios-3.15.0-pythonorg, and PBS desktop (macOS arm64/x86_64, Linux x86_64, Windows x86_64) in both GIL and freethreaded;
   • remove the 3.15.0rc1 entries;
   • compute every SHA-256 from the downloaded file, and check it against PBS's SHA256SUMS and python.org's sigstore bundles.
3. Confirm the rc-era branches (pythonOrgReleaseDir, iosFromPythonOrg) behave for a final release. The 3.14 GIL path stays.
4. Research only: is there a PEP 783 ABI for 3.15 (wasm stays on CPython 3.14.2 / pyemscripten_2026_0)?
5. Verify, each in its own invocation (AGENTS §15):
   • :python-multiplatform:compileKotlinAndroidNativeArm64
   • :python-multiplatform:compileKotlinIosSimulatorArm64
   • :python-multiplatform:desktopTest
   • the three fixture modules

Done when those pass with counted results.