feat: add experimental external-outcome evidence vectors and validator - #181

Builds on #180.

This PR no longer claims to introduce a novel outcome-verification protocol.

A standards check found substantial existing work:

• AEB -07 already defines authoritative terminal evidence, EXECUTED / FAILED / INDETERMINATE, and authenticated reconciliation.
• SCITT AI-Agent Action Receipt -01 already separates dispatch, controller-reported success, and physical-effect evidence.

The remaining value here is executable interoperability and conformance evidence.

This PR therefore provides:

• JSON Schema for an experimental CounterProof outcome-evidence vector;
• explicit non-claims;
• a physical-window contradiction fixture;
• a validator that rejects incomplete or stale evidence from supporting VERIFIED / CONTRADICTED;
• tests proving that an executor acknowledgement alone is not enough;
• a deliberately narrow role: concrete vectors that can be mapped into or used to test existing external specifications.

The artifact is not intended to graduate into a CounterProof-owned standard merely because CI passes. Its success condition is an external implementation, draft, or workflow consuming/reproducing the vectors.

The high-value next case is a real hardware mismatch: executor acknowledgement succeeds while fresh authoritative readback contradicts the intended physical state.