Fix registry and MCP safety gaps

[cursor]

Bug and impact

• Registry fetch could crash after accepting a valid remote snapshot when the local SSOT was corrupt or quarantine staging failed, instead of returning a fail-closed result.
• Corrupt, empty, or sparse generated threat-registry projections could silently disable packaged detector rules.
• Projection generation dropped semantic_aliases and mythic_framing records.
• MCP-exposed JSON/raw inputs could be parsed or stripped before shared size limits, creating resource-exhaustion paths.
• A symlinked registry_fetch/ quarantine layout could expose, purge, or receive writes outside the quarantine root.
• Fresh installs could resolve MCP 2.x even though this code imports the MCP 1.x FastMCP API.

Root cause

Recent registry/quarantine hardening added new layout and projection paths, but several failure boundaries still trusted first-found local files, followed symlinked layout directories, or parsed unbounded MCP strings before applying existing limits. Package metadata also lacked the MCP <2 compatibility bound.

Fix

• Return fail-closed registry fetch errors for corrupt SSOT and quarantine write failures.
• Skip invalid/empty registry candidates and overlay generated projections onto packaged rules.
• Preserve semantic_aliases and mythic_framing in generated projections.
• Apply SCP_MAX_INPUT_CHARS before MCP/core JSON parsing and raw contribution stripping.
• Reject or skip unsafe symlinked registry-fetch layout dirs in quarantine write/list/purge/quota paths.
• Bound MCP dependency to mcp>=1.2.0,<2.0.0.

Validation

• RED/GREEN regressions added for each concrete trigger.
• python3 -m compileall -q src/scp && git diff --check && PYTHONPATH=src python3 -m pytest tests/test_registry_fetch_r4.py tests/test_registry_load_order_r5.py tests/test_pattern_record_r1.py tests/test_security_regressions.py tests/test_appsec_antigen_mcp_gates.py tests/test_registry_contribute_r6.py -q -> 88 passed.
• PYTHONPATH=src python3 -m pytest -q -> 367 passed / 3 skipped / 2 failed; failures are pre-existing vendored contract hash mismatches for scp_mcp_v1 and scp_mcp_v1.1.

  

[ManintheCrowds]

Audit: symlink layout + quarantine write fail-closed + related registry guards covered by #60. Projection-bucket work already on main via #43. MCP size-limit / contribute extras not required for this consolidation. Closing as superseded by #60.