Fix registry contribution and SSOT safety gaps

[cursor]

Bug and impact

• Structured patterns_json contribution could carry raw prompt text in source_ref metadata and publish it in the registry snapshot when approve=true and consent/host allowlist were configured.
• Deeply nested URL-encoded override phrases could be decoded only to reversal tier instead of injection, so scp_validate_output would still mark them safe.
• Registry SSOT writes were direct and committed before projection persistence; a failed projection write could leave SSOT updated while inspect continued using the stale projection, and corrupt SSOT JSON could crash fetch/diff paths.

Root cause

• The contribution gate closed only top-level fields and did not constrain source_ref keys/values.
• Canonicalization was capped at four layers.
• SSOT/projection persistence used direct writes and committed SSOT before projection write success.

Fix

• Reject unknown structured contribution detector/source-ref fields and constrain source_ref.lang to language-tag metadata.
• Increase bounded canonicalization depth to catch five-layer URL encoding while retaining a hard cap.
• Load corrupt SSOT fail-closed, write JSON via temp-file replace, and persist projection before committing SSOT.

Validation

• PYTHONPATH=src python3 -m pytest -q tests/test_registry_contribute_r3.py::test_structured_patterns_rejects_unknown_detector_field tests/test_registry_contribute_r3.py::test_structured_patterns_rejects_unabstracted_source_ref_metadata tests/test_registry_contribute_r3.py::test_structured_patterns_rejects_unabstracted_source_ref_lang_value tests/test_sanitize_encoding_evasion.py::TestComposition::test_deep_url_encoded_injection_classified tests/test_registry_ssot.py::test_load_ssot_returns_empty_for_corrupt_json tests/test_registry_ssot.py::test_apply_merge_does_not_commit_ssot_when_projection_write_fails -> 6 passed
• PYTHONPATH=src python3 -m pytest -q tests/test_registry_contribute_r3.py tests/test_sanitize_encoding_evasion.py tests/test_registry_ssot.py tests/test_registry_load_order_r5.py tests/test_registry_fetch_r4.py tests/test_appsec_host_allowlists.py -> 116 passed
• PYTHONPATH=src python3 -m pytest -q with .[dev,antigen] installed -> 286 passed, 3 skipped, 3 known baseline contract hash failures in tests/test_contract_document_hash.py