fix(crosscheck): isolate locked Python proof dependencies

[ruby-dlee]

Summary

• prepare exact-head Python proof dependencies from tracked pyproject.toml and uv.lock into bounded private state before execution
• keep preparation and proof execution no-network, deny proof access and writes to audited cache/reviewer/ambient roots, and record lock/environment provenance
• fail closed on missing, stale, malformed, oversized, raced, or otherwise unusable inputs without ambient fallback

Evidence

• reproduces the fresh-cache offline collection failure before its named control
• exercises the production Crosscheck lifecycle so exact finding cc-aaaaaaaaaaaa reaches verified-fixed only after baseline pass and mutation failure
• executes denial probes with positive controls for cache reads/writes, reviewer and ambient environment access, and network
• preserves existing exact-head, reviewer-independence, lifecycle, and mutation-proof coverage

Validation

GitHub CI is the authoritative 8-shard behavior-suite gate for this expedited landing. The branch head submitted here is b64e000454c3bba1515c50f563d11e30954c9e96.