Add corpus-event JSON-LD/YAML serialization fixtures

[mdheller]

Context

Follow-up from #89 after #103 merged the RDF/SHACL corpus-event semantics tranche.

#103 implemented:

• Platform/corpus-event-semantics.ttl
• shapes/corpus-event-semantics.shacl.ttl
• valid corpus-event Turtle fixture
• invalid fixtures for missing provenance, missing confidence, and missing evidence
• focused validator and make validate-corpus-event-semantics
• documentation and supplemental registry

#89 also requested that schemas should be serializable to JSON/YAML. That portion was not implemented in #103 and is split here so #89 can close without losing the residual.

Scope

Add serialization fixtures for corpus-event semantics objects:

• JSON-LD context or explicit JSON-LD example fixture for corpus-event objects
• JSON example fixture where appropriate
• YAML example fixture where appropriate
• roundtrip or validation notes showing how downstream consumers should serialize/deserialize the core classes

Required object coverage

At minimum cover:

• CorpusEvidenceBundle
• SourceProvenance
• EventSchema
• EventInstance
• CausalRelationCandidate
• SemanticTableAnnotationTask
• ColumnOntologyLink
• DiagnosticFinding

Acceptance criteria

• JSON-LD/JSON/YAML examples exist or intentionally scoped subset is documented.
• Examples map back to the existing Platform/corpus-event-semantics.ttl vocabulary.
• Validation path is documented.
• make validate remains green.

Non-goals

• No graph runtime.
• No KGQA runtime.
• No external bibliography harvesting.
• No Sherlock retrieval implementation.
• No Policy Fabric decision runtime.
• No Agentplane execution surface.