F-097: first-class signing configs on androidBinary

[stepango]

Summary

• First-class APK signing on androidBinary only (GH Support build types #51 remaining checkbox / F-097).
• New FormaSigningConfig + signingConfigs / buildTypeSigning attrs; wired to AGP ApplicationExtension.signingConfigs and build-type assignment (name to name bridge).
• Sample dummy keystore (application/binary/demo-release.keystore); docs (CALL-SITE-SURFACE APK signing section); :android unit tests (7).
• Libraries keep BuildConfiguration build-types only — no per-module signing shopping.

Verify

• plugins/: :android:test (7 passed) + jacocoHappyPathCoverageVerification → SUCCESS
• application/: :binary:assembleDebug + :binary:assembleRelease → SUCCESS

Closes

Closes #51