Fix registry quarantine provenance and MCP dependency crash

[cursor]

Bug and impact

• A forged .json file under the registry_fetch quarantine layout could act as both the registry snapshot payload and its own sidecar metadata. With operator approval/consent, apply_merge() accepted it and merged untrusted pattern records, bypassing the intended requirement that merge inputs are .txt content files produced by scp_fetch_registry.
• Fresh installs resolved mcp 2.x, but scp.antigen_mcp imports the MCP 1.x mcp.server.fastmcp API, causing Antigen MCP import/startup failure.

Root cause

• _load_quarantine_snapshot() verified the path was under registry_fetch/ and that sidecar reason=registry_fetch, but did not require the merge input path to be a .txt content file. For a .json payload, with_suffix('.json') pointed back to the same file.
• pyproject.toml declared mcp>=1.2.0 without an upper bound despite the code using the pre-2.0 FastMCP API.

Fix

• Reject non-.txt registry-fetch quarantine merge inputs with quarantine_provenance_rejected.
• Add a regression test for self-certifying .json payloads.
• Constrain the runtime dependency to mcp>=1.2.0,<2.0.0.

Validation

• RED: PYTHONPATH=src python3 -m pytest tests/test_registry_ssot.py::test_apply_merge_rejects_json_payload_self_certifying_as_sidecar -q failed before the fix because the forged payload merged.
• GREEN: PYTHONPATH=src python3 -m pytest tests/test_registry_ssot.py tests/test_registry_fetch_r4.py tests/test_registry_load_order_r5.py tests/test_appsec_antigen_mcp_gates.py -q -> 47 passed.
• python3 -m compileall -q src/scp && git diff --check -> passed.
• Full PYTHONPATH=src python3 -m pytest -q was also run: 351 passed / 3 skipped / 7 failed. The failures were existing unrelated baselines: contract hash mismatches, RT11 residual, and optional coincurve setup tests without antigen extras.

  

[ManintheCrowds]

Audit: self-certifying sidecar reject in #60; MCP pin skipped. Closing as superseded by #60.