fix(persist): complete the storage boundary; respect binary content

[wolfy-j]

Audit follow-ups to #67:

• The UTF-8 boundary now covers every persist write: CREATE/UPDATE data content and metadata, node config and metadata, dataflow metadata, iteration-terminal replacement, commit payloads and metadata (ops, commit, commit_repo).
• Content declared application/octet-stream passes byte-identical — sanitizing binary is corruption, so the boundary is content-type aware (ensure_storable).
• A terminal table result that fails json.encode (cycles) persists as its failure text instead of poisoning the completion batch; the test uses a genuinely non-encodable value.
• The execution-identity contract definition had zero consumers and zero bindings since the app-side retirement; removed with its harness stubs.

Tests: 968 sqlite green.