Harden audit core: fail-closed compare and align trust semantics

[aelitium-dev]

Summary

This PR hardens the audit core and aligns trust semantics with implemented verifier behavior.

Changes

• make compare fail closed on invalid bundles
• return INVALID_BUNDLE when verification fails before comparison
• remove model-causality wording from compare interpretation output
• align TRUST_MODEL.md with current verifier semantics
• clarify that the verifier checks byte/hash integrity and minimum operational structure, not full canonical payload schema correctness

Validation

• run_test_matrix.sh: PASS on A and B
• unittest: PASS on A and B
• tampered compare case now returns STATUS=INVALID_BUNDLE rc=2

Notes

This PR does not expand architecture or trust boundary.
It hardens behavior within the current implemented surface.