Add frozen CH06 conformance matrix

[404SecNotFound]

Cohaera's CH06 evidence-integrity claim had unit and smoke coverage but no frozen comparison against a verifier that does not share its implementation. This PR adds a deterministic conformance lab built from one pristine signed, multiplexed stream and applies nine mutations only after signing.

The lab covers intact, deleted, modified, chain-metadata-stripped, reordered, truncated, replayed, missing-key, and unsupported-scheme inputs. A hand-written expectation file is compared with both Cohaera and a standard-library-only baseline that independently implements canonical hashing, sequence/chain checks, Ed25519 verification, and replay state.

Current result:

• 9/9 predeclared cases pass.
• 0 manipulated affected sessions report verified_complete.
• 0 intact or reorder-only sessions report inadmissible.
• Missing and unsupported evidence explicitly degrades or declines.
• Cohaera provides additional per-session localization for a body modification and a truncated stream.

Validation:

• python lab/local/run.py --check
• python lab/ch06/run.py --check
• pytest -q tests/test_ch06_lab.py tests/test_lab.py — 48 passed
• python tools/verify.py — 16 passed, 0 failed, 0 not evaluated; 1,190 tests