Add abstraction-verification artifacts for program candidates and counterexamples

[mdheller]

This PR adds the first abstraction-verification artifacts to the semantic contract layer.

Why:

• abstract reasoning needs explicit artifacts for program candidates and counterexamples
• compile success and semantic correctness must be represented separately
• downstream policy and runtime gates need stable semantic refs for verification evidence

What this PR adds:

• ProgramCandidateArtifact schema
• CounterexampleArtifact schema
• example payloads for both artifact families

What this PR does not do:

• it does not yet patch canonical_enums.yaml
• it does not yet add the full planning artifact pack in one shot