Prefix OO-LD keywords with x-oold-, add dialect meta-schema and CI validation

[simontaurus]

Implements decision 6 (prefixing, #42) and decision 4 (meta-schema + optional vocabulary, #41) from the gap analysis (#18), and adds the CI validation job (#23).

#42 - prefix all OO-LD-proprietary keywords with x-oold-

Every OO-LD custom keyword is prefixed, so it is a valid JSON-SCHEMA extension keyword and a valid OpenAPI 3.0 Specification Extension:

• range -> x-oold-range
• title* / description* -> x-oold-multilang-title / x-oold-multilang-description
• iri -> x-oold-iri
• backward-compatible-with -> x-oold-backward-compatible-with; incompatible-with -> x-oold-incompatible-with; prose oold-prior-version -> x-oold-prior-version
• Already conforming (unchanged): x-oold-ref, x-oold-reverse-*, x-oold-uuid, x-oold-version
• Left unchanged: @context (JSON-LD keyword), the name* multi-mapping (postponed, Extended JSON-LD context notation for synonyms #12), third-party json-editor keywords (format, defaultProperties), and the LinkML mapping example's own range:.

#41 - OO-LD dialect meta-schema + optional $vocabulary

• New meta/oold-meta-schema.json: extends the 2020-12 meta-schema, re-lists the seven standard vocabularies as required (Core 8.1.2.2), declares the OO-LD vocabulary optional (false), and gives each x-oold-* keyword a syntax constraint and a description.
• New README section "Meta-schema and vocabulary".
• $id and the OO-LD vocabulary URI are provisional (flagged in the file) pending the hosting decision.

#23 - CI validation

• New examples/ (Thing <- Person, Organization). Examples use a relative $id so the base URI is the retrieval location: the same relative $ref resolves both locally (file path) and once deployed (URL).
• New scripts/validate.mjs + package.json and workflow .github/workflows/validate-schemas.yml running npm install && npm run validate, which checks: the meta-schema is valid 2020-12, each example is a well-formed OO-LD schema, and each example's standard $ref composition resolves offline.

Notes

• x-oold-range's subschema form is left permissive and does not yet validate x-oold-reverse-* inside it.
• The x-oold-range section still uses $ref internally; the $ref -> x-oold-ref redesign is Type-range of properties formalization #20.

Acceptance criteria

#42:

• OO-LD custom keywords prefixed x-oold-*
• @context -> REST-API-LD x-jsonld-* derivation documented (follow-up)
• Delivery-tier table added to README (follow-up)

#41:

• OO-LD dialect meta-schema published + example validates against it
• $vocabulary declared optional; generic 2020-12 validator still processes OO-LD schemas
• x-oold-* keywords covered by the meta-schema syntax