test(conformance): cover multi-hop action evidence boundaries

[10adnan75]

What

Add MUST-level conformance coverage for the remaining delegation-linked action-evidence boundaries:

• valid, strictly attenuating multi-hop delegation;
• intermediate scope widening classified as provenance-invalid;
• delegatee/subject mismatch classified as provenance-invalid.

The new cases are ACTION-009 through ACTION-011.

Why

Follow-up to #36. This PR covers the residual action-evidence cases not addressed by #76, which adds invalid-signature coverage as ACTION-008.

It verifies that action evidence preserves the existing distinction between valid provenance, provenance-invalid evidence, and authorization-invalid outcomes.

Security impact

None to production runtime behavior. This is conformance-test and documentation coverage only. It strengthens regression protection for the fail-closed delegation-chain invariants: scope attenuation and provenance-to-authority binding.

Test plan

• pytest passes
• ruff check passes
• mypy passes
• Manual test performed

Additional automated validation::

• Ran the conformance suite and confirmed ACTION-009 through ACTION-011 pass.
• Ran the full unit suite: 221 passed, 3 skipped.
• Ran Bandit with no issues identified.

DCO sign-off

• I certify that I wrote or have the right to submit this contribution, and I agree to the
  Developer Certificate of Origin (https://developercertificate.org).

[imran-siddique]

Reviewed the diff. Test and documentation only, no runtime change, and the coverage is real rather than self-referential: _verify_action_evidence drives the actual library (verify_delegation_chain, verify_dag, cross_check_chain, handle_peer_request), so these assert implementation behaviour, not a test-local reimplementation.

The three cases are the right residual boundaries:

• ACTION-009 pins that strict attenuation across three hops still verifies, which is the positive control the other two need.
• ACTION-010 puts the widening at the intermediate hop rather than the leaf, which is the case a leaf-only check would miss.
• ACTION-011 breaks subject-to-credential binding while leaving the hash chain intact, so it isolates cross_check_chain rather than incidentally tripping verify_dag.

Numbering leaves ACTION-008 for #76, which is the right call.

One merge-order note for whoever lands these: this and #76 both append to the same README table and the same tail of test_profile_conformance.py. Each is mergeable against main today, but the second one in will need a rebase.

Only failing check is the maintainer-hold gate.

[imran-siddique]

This is approved but currently conflicting with main, so it cannot merge yet. A rebase onto current main is all it needs. The approval stands once it is clean.

[10adnan75]

Rebased onto current main after #76 and resolved the shared action-evidence append points.

ACTION-008 is retained from #76; this PR adds ACTION-009 through ACTION-011.

The refreshed conformance suite passes: 34 passed.

@imran-siddique Could you please re-approve the current head commit so the maintainer-approval gate can clear?

[imran-siddique]

Re-approving the rebased head. Same content I approved before, and the rebase resolved the shared append points cleanly. ACTION-009 through ACTION-011 line up with the README table. Thanks for the quick turnaround.

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