Fix registry fallback and MCP dependency crashes

[cursor]

Summary

• Skip corrupt or empty threat-registry projection files and fall back to the packaged registry so registry-backed detections stay active.
• Pin MCP to the FastMCP-compatible v1 line in pyproject and requirements to prevent import-time crashes under MCP 2.x.

Validation

• RED: PYTHONPATH=src python3 -m pytest tests/test_registry_load_order_r5.py::test_bad_default_projection_falls_back_to_packaged_registry -q failed before the loader fix.
• RED: PYTHONPATH=src python3 -m pytest tests/test_mcp_dependency_bounds.py -q failed before the dependency bound.
• GREEN: PYTHONPATH=src python3 -m pytest tests/test_mcp_dependency_bounds.py tests/test_registry_load_order_r5.py tests/test_pattern_record_r1.py tests/test_registry_fetch_r4.py tests/test_registry_ssot.py -q -> 38 passed.
• python3 -m compileall -q src/scp
• git diff --check HEAD~2..HEAD
• python3 -m pip install -q -e '.[dev]' then FastMCP import smoke -> fastmcp import ok.
• Full suite after compatible MCP install: 377 passed / 3 skipped with known baseline failures in contract hashes, RT11 residual, and missing optional coincurve tests.