ci: gate LeanTypeDual fork invariants

[AsafMah]

Summary

• add a fast source/configuration gate for LeanTypeDual identity, versioning, flavors, privacy manifests, retained fork integrations, dictionary packaging, and release workflow coverage
• add a packaged-APK gate for the exact four release artifacts, effective IDs/minSdk/INTERNET policy, and recursive dictionary contents
• run the source gate in unit CI and the artifact gate after all four signed release APKs are assembled
• add fixture-driven mutation coverage and document both commands

Validation

• python tools/check_fork_invariants.py
• python -m unittest discover -s tools/tests -v (34 tests)
• Python compilation, workflow YAML parsing, and git diff --check

Implementation tradeoff

The source gate is intentionally a fast, strict prefilter using stdlib structural parsing. Release CI provides the authoritative packaged check through apkanalyzer plus ZIP asset inspection without making every PR assemble four APKs.