Mark residual fallbacks as boundaries

[yha9806]

Summary

• classify fallback residuals with boundary status and owner
• expose expected/unexpected fallback counts in residual audit reports and CLI output
• lock the current recovered residuals as runtime provider recovery vs visual ownership planner boundaries

Verification

• PYTHONPATH=src python3 -m pytest tests/test_fallback_residual_audit.py -q
• PYTHONPATH=src python3 scripts/real_source_context_recovery_eval.py --repo-root . --case-source-manifest docs/benchmarks/learning/combined_case_source_manifest_v1.json --artifact-search-root /Users/yhryzy/dev/vulca/.scratch/p2b-live-shipgate --image-search-root /Users/yhryzy/dev/vulca/.scratch/p2b-live-shipgate --source-dependency-manifest docs/benchmarks/learning/real_source_dependency_label_manifest_v1.json --output-dir build/agent_runtime_boundary_v1/real_recovery_eval --report build/agent_runtime_boundary_v1/real_recovery_eval/report.json --max-recovered-source-context-gaps 0 --min-fallback-agent-reduction 2 --min-recovered-eval-cases 2
• PYTHONPATH=src python3 scripts/fallback_residual_audit.py --decision-path build/agent_runtime_boundary_v1/real_recovery_eval/recovered/dry_run/dry_run_decisions.jsonl --report build/agent_runtime_boundary_v1/residual_audit/report.json
• PYTHONPATH=src python3 -m pytest tests/test_fallback_residual_audit.py tests/test_real_source_context_recovery_eval.py tests/test_training_effectiveness_report.py tests/test_source_context_gap_pack.py tests/test_dry_run_decision_router.py -q
• ruff check src/vulca/learning/fallback_residual_audit.py scripts/fallback_residual_audit.py tests/test_fallback_residual_audit.py
• python3 -m py_compile src/vulca/learning/fallback_residual_audit.py scripts/fallback_residual_audit.py
• git diff --check