Resolve schema references through their Source

[malberts]

For #993 (work order T3). Builds on the T2 Source
contract PR and targets its branch, so the diff shows only this track.

Schema references become (source, name), resolved through the schema's own Source and degrading
gracefully when unresolvable. Reference plumbing only; no non-local schema source exists yet.

Design calls recorded here:

• A distinct SchemaReference value object with SchemaName intact underneath, per the work
  order's own lean: locally the name IS the page title in the Schema namespace, and a
  source-qualified reference is not a valid title.
• Foreign references serialize as a JSON object { "source": ..., "name": ... }, not a
  source:Name string. Schema names are page titles and may legitimately contain colons, so a
  string-concatenation grammar is ambiguous; subject ids avoid that only because the bare nanoid
  alphabet excludes the separator. Local references stay bare name strings, byte-identical to
  before (regression-pinned on both persisted spots).
• Canonicalization: on the subject schema field, an object naming the local Source normalizes
  to the bare form at parse time (mirroring SubjectIdParser), so one Schema is never referenced
  under two identities. A relation property's targetSchema is deliberately not at full parity
  yet: schema save validation still accepts only the bare string form, so the object form has no
  ingress there, and normalization is omitted where it can never run. Widening the schema-save
  contract and canonicalizing that path belongs with sourced-schema authoring; flip it in review if
  you prefer parity now.
• Resolution routes through the T2 registry via SourceRoutingSchemaReferenceResolver,
  mirroring the subject routing semantics exactly: unknown source keys degrade to null plus one
  logged warning, and unresolvable references land in the pre-existing schema-not-found /
  null-schema handling in all five converted consumers (validators, replace action, RDF projector,
  Neo4j updater), never fatally. Name-parameterized paths (REST schema APIs, new-subject creation)
  stay name-based.
• The Neo4j schema-name node label stays local-only (work-order constraint), and the frontend
  reduces string-or-object references to the bare name: sourced rendering is deferred by ADR 23, so
  the TS domain keeps plain names end to end.

Docs: docs/api/subject-format.md gains the Schema References section, docs/api/schema-format.md
documents the targetSchema forms and their current acceptance honestly, and the glossary Schema
entry notes the reference form.

_{AI-authored — Claude Code, Fable 5 (max); standing directive from @malberts to complete the #993 tracks without blocking on inter-track human review, design calls delegated; diff not yet human-reviewed; TDD with watched failures per behavior, two adversarial AI review lenses with live probes (hook-ordering re-trace across five accessor orders, byte-identity on seeded content, ingress enumeration for the canonicalization asymmetry), make ci and make ts-ci green locally, branch CI pending at posting time.}

Production notes

Design and review adjudication by Fable 5 (max); implementation by an Opus 4.8 (max) subagent;
two Opus 4.8 (max) review lenses (runtime correctness with container probes, tests/conventions/
docs) stood in for the usual pre-PR human review at @malberts' direction. The review confirmed the
targetSchema asymmetry is unreachable through every current ingress and found only docs overclaims,
which the final commit scopes to shipped behavior.