feat(crosscheck): stage bounded review snapshots

[ruby-dlee]

Summary

• build a deterministic, bounded exact-head Git-blob snapshot before Azure lane admission or paid compute
• bind the snapshot, manifest, base SHA, head SHA, exclusions, and bounded base guidance into the reviewer identity
• safely download, verify, and extract the snapshot read-only in the isolated model guest
• stage and clean the snapshot through the existing exact-blob lifecycle with nested optional telemetry

Parent-failing regression evidence

Against cc-2a, the new regression harness failed because the exact-head snapshot builder and the guest snapshot parameter contract did not exist. The release adds executable coverage for deterministic construction, binary and oversized exclusions, changed-file allowances, symlinks, hardlinks, traversal, aggregate and manifest bounds, giant-blob streaming avoidance, preflight normalization, non-UTF8 paths, base identity, guidance tampering, and tracked metadata shadowing.

Validation

• bash tests/fm-crosscheck.test.sh
• bash tests/fm-crosscheck-azure.test.sh
• direct ledger compatibility tests
• Python compilation and shell syntax checks
• adversarial review: all reported P0-P2 issues fixed and regression-covered
• real Relvino snapshot: 7,194 files, 284 bounded exclusions, 169,804,332 uncompressed bytes, 38,217,010 compressed bytes, 145.872s build, 2.077s extraction, read-only extraction verified

Size justification

The change is intentionally one release unit because the producer, guest verifier, lifecycle cleanup, identity contract, documentation, and executable security regressions form one atomic snapshot boundary. Splitting them would temporarily permit an unverified or uncleared snapshot contract.

Live checkpoint

• validation PR: chore(validation): prove identity-only Crosscheck review #338, exact head e616a585347e4713d1721bfe6ddec16c86d0c668
• task: cc-2b1-v1-e616a585-20260826b
• result: CLEAR, official verification returned the exact head
• mode: Azure compartments, identity-only, zero proof compartments
• cleanup: model and staging both complete
• wall time: 380.9 seconds

The disposable validation PR was closed without merging after this checkpoint. This stacked PR is not merged here.