[Feature]: Migrate SchemaVerifier to DiagnosticResult (Phase 4)

[rahuldass19]

Purpose

Migrate SchemaVerifier (schema_verifier.py) to return DiagnosticResult. JSON schema validation is already deterministic — this is a straightforward mechanical migration.

Related

• Meta: META: DiagnosticResult engine conformance — full migration plan (all 13 engines) #216 (Phase 4 — SQL + Schema + Stats engines)

Scope

1. Remove orphan code (audit finding from #216)

schema_verifier.py:100-109,607-660 instantiates a SymbolicVerifier as math_verifier but never calls it. It performs inline float comparison instead. This dead delegation should be cleaned up during migration.

2. Migrate return type to DiagnosticResult

• verify_schema() → DiagnosticResult
• validate_schema() → DiagnosticResult
• All statuses map to VERIFIED / UNVERIFIABLE / BLOCKED
• agent_message sanitized (no raw jsonschema output)

3. proof_ref for schema validation

proof_ref = compute_proof_ref(schema_evidence) when verification succeeds — the validated schema + instance are the proof artifact.

Status Mapping

Current state	DiagnosticResult
Schema valid	VERIFIED with proof_ref
Schema invalid	VERIFIED with developer_fields identifying violation
Parse error (schema)	BLOCKED, constraint_id: schema_verifier.parse_error
Validation error	BLOCKED, constraint_id: schema_verifier.validation_error

Success Criteria

• All public methods return DiagnosticResult
• Orphan math_verifier code removed
• proof_ref present on VERIFIED, null on UNVERIFIABLE/BLOCKED
• agent_message sanitized
• Regression tests assert on DiagnosticResult fields