Clarify $uses placement and effective-schema processing

[clemensv]

Problem

The Core description of $uses is not sufficiently precise for interoperable instance validation.

It says $uses is available only in instance documents, while also saying that it may be used in a meta-schema referencing a parent schema. Its value is described as add-in names or JSON Pointers, but the placement, lookup context, failure behavior, and effect on the root effective schema need a single normative algorithm.

This matters for closed root objects: $uses is document-control metadata, but accepting it without applying and validating the selected add-ins silently under-validates the instance.

Decisions required

1. Define every document kind and location where $uses is permitted.
2. Define the allowed value shape, uniqueness requirements, and whether an empty set is valid.
3. Define resolution of offered names and same-document JSON Pointers for each permitted document kind.
4. Require an error for unknown, malformed, conflicting, or inapplicable add-ins.
5. Define how selected add-ins produce the effective root schema, including properties, required, and additionalProperties.
6. State that $uses is excluded from application-property matching only after its own validation and application.
7. State whether validation APIs may mutate the input instance while processing $uses.

Acceptance criteria

• Core provides a deterministic $uses processing algorithm.
• Root-only and meta-schema cases are unambiguous.
• Closed-root examples cover valid, unknown, malformed, and conflicting add-ins.
• The Core meta-schema and SDK conformance assets reflect the resolved rules.