feat(csharp): add C# analyzer support

[duriantaco]

Summary

• add a C# language visitor with symbol extraction, private-method dead-code support, and selected security sink detection
• wire .cs discovery into analyzer summaries, deep-audit candidates, polyglot audit signals, and benchmark language support
• update CLI/README language support plus the generated repo map

Validation

• pytest -q test/test_csharp_security.py test/test_analyzer.py::TestAnalyze test/test_security_benchmark.py test/test_dead_code_benchmark.py test/test_audit_candidates.py test/test_audit_processor.py test/test_audit_ci.py test/test_audit_export.py
• python scripts/build_repo_map.py --check
• git diff --check
• staged Skylos gate passed during commit
• push parity check passed: 32 passed, 27 deselected

Notes

• no new rule IDs were introduced; C# findings reuse existing SKY-D rule IDs
• scripts/check_rule_docs_parity.py still fails on existing catalog/docs drift unrelated to this branch