fix(provenance): validate attestation by shape, not truthiness

[rajnisht7]

What this changes

provenance._check_structure() checked attestation with truthiness (not attestation / attestation) instead of checking its type and shape. Spec (server-provenance-v1.md §3) says attestation must be null unless kind == "tee-attested", and must match the TRACE v0.2 §3.1 runtime shape when it is.

Truthiness doesn't enforce that:

• {}, [], "" are falsy but not null, so a non-TEE record carrying any of them as attestation built and verified cleanly.
• Any non-empty string/list/dict is truthy, so a tee-attested record with attestation: "hello" or ["anything"] passed too.
• Even a schema-valid attestation could name platform: "software-only", which is not a hardware claim at all the same contradiction §1 rules out for tee-attested ("the server itself, from inside a TEE").

Because this check is shared with verify_record() (#146), a hand-forged, correctly-signed record with attestation: "not-actually-attestation" passed verification.

Type of change

• Editorial (typo, link fix, clarification: no normative effect)
• Non-breaking spec change (new optional field, new platform profile, informative addition)
• Breaking spec change (requires 14-day comment period and Project Lead sign-off)
• Schema change
• Example addition

Spec section

Not a change to spec/trace-v0.2.md itself this implements the existing server-provenance-v1.md §3 attestation contract and TRACE v0.2 §3.1 runtime shape more strictly in code. No wire format or normative text changes.

Checklist

• DCO sign-off on all commits (git commit -s)
• CHANGELOG.md updated (for any normative change)
• Breaking changes marked with <!-- CHANGED: #NNN: description --> in spec text
• Backward compatibility statement included (for breaking changes)