Retarget the normative JSON-Schema dialect from Draft 4 to 2020-12

[simontaurus]

Problem

The spec declares Current support covers [Draft 4] (README L426), but it relies on constructs that are invalid under Draft 4:

• $ref alongside sibling keywords. OO-LD places $ref next to type/properties/range/@context, and allOf: [{$ref: ...}] next to properties (Composition). This is only legal from JSON Schema 2019-09 onward. Core §8.2.3.1: "Note that this definition of how the results are determined means that other keywords can appear alongside of $ref in the same schema object." Under Draft 4/7 these siblings are ignored.
• const (e.g. "type": {"const": "schema:Organization"}) was introduced in draft-06 and is not a Draft 4 keyword (Validation §6.1.3), so the current spec is internally inconsistent.

Resolution

• Declare "$schema": "https://json-schema.org/draft/2020-12/schema" as the OO-LD base dialect; update README L426.
• State that OO-LD composition requires 2019-09+ $ref-with-siblings semantics.
• List Draft-4 idioms to avoid if present: definitions -> $defs, id -> $id, boolean exclusiveMinimum/exclusiveMaximum -> numeric.

Note (tooling reality)

Empirically, ajv 8 and json-editor 2.17.1 both evaluate const and $ref-siblings even under an explicit draft-07 $schema, so this is primarily a spec-correctness fix rather than a breakage fix.

Acceptance criteria

• README declares JSON Schema 2020-12 (Target JSON-SCHEMA 2020-12 as the normative dialect #43)
• All examples validate against the 2020-12 meta-schema (Prefix OO-LD keywords with x-oold-, add dialect meta-schema and CI validation #44)
• Draft-4 -> 2020-12 idiom migration list documented (Target JSON-SCHEMA 2020-12 as the normative dialect #43)

Source: gap analysis #18.

[simontaurus]

Dialect retarget to JSON-SCHEMA 2020-12 landed in #43; example schemas are now validated against the 2020-12-based OO-LD meta-schema in CI (#44).