mdcode: add OWL export (OSI -> Turtle), the round-trip inverse of owl import

[libei]

What

Adds kcmd owl export <model.yaml> — the reverse of kcmd owl import. It writes a semantic model back out as a Turtle (.ttl) OWL ontology, the exact mirror of the import pipeline:

• from_ir.ts — IR → staging OwlModel (the inverse of to_ir.ts)
• serialize.ts — OwlModel → Turtle (the inverse of parse.ts)
• convert.ts — new convertOsiToOwl() orchestrator; commands.ts/main.ts gain the export action

Scope: round-trip fidelity

This was scoped deliberately (not a general "model → OWL" claim). A model that originated as OWL exports losslessly: OWL → OSI → OWL → OSI is stable at the IR level. It:

• maps every native construct back (class → owl:Class, field → owl:DatatypeProperty, relationship → owl:ObjectProperty, primary_key → owl:hasKey, single-col unique_key → owl:InverseFunctionalProperty, extends → rdfs:subClassOf, datatype → its xsd: range);
• re-emits the carried GOOGLE owl:/rdfs: extensions verbatim (owl:inverseOf, rdfs:subPropertyOf, property characteristics, per-term annotations, …), rebuilding a shortened in-namespace name to a full IRI from the carried owl:baseIri;
• reproduces each entity's field order via a topological merge of per-entity orders (so multi-domain properties land correctly).

A model authored natively can hold things OWL has no shape for — a metric, a field whose expression is not a bare column, an importedExpression, a bound source, an M:N association, a composite unique_key, deployment targets. Those are dropped with a warning, never misrepresented; the class/property/edge they hang off is still exported.

Tests

• owl_export.test.ts: all five OWL import fixtures round-trip OWL → OSI → OWL → OSI unchanged (zero warnings); sales is pinned to a golden sales.owl.golden.ttl; the golden re-imports to the same model; and each non-OWL construct is verified to warn + drop.
• Full test:semantic suite (488 tests) green; verified end-to-end through the real CLI (kcmd owl export → kcmd owl import round-trips).

Docs

owl-import.md gains an Exporting to OWL section and the reverse-direction limitation is updated; README.md mentions the new command.