feat(registry): validate link columns

[msto]

Summary

Related to #42. Stacked on #51. Tracked at #53.

Extends _compare_unwrapped with a dispatch branch for
LatchRecordModel subclasses, plus _compare_link.

• Per spec, the link's target table is not checked — any
  LatchRecordModel subclass matches any link column, keeping the
  check tolerant of models that represent only a subset of a larger
  linked table. The SDK's to_python_type maps link columns to
  Record, so a column is a link iff column_type is Record.
• _looks_like_latch_record_model is a structural check (per
  your review): instead of issubclass(t, LatchRecordModel) —
  which would force a lazy local import to avoid the circular
  _record_model ↔ _schema dependency — it verifies that t is a
  Pydantic BaseModel subclass with the id and name fields
  LatchRecordModel mandates. A dedicated test confirms an unrelated
  BaseModel without those fields is NOT treated as a link.

Split out of the original combined array+link PR per your request.

Test plan

• Happy path with a typed LatchRecordModel subclass.
• Happy path with the bare LatchRecordModel base.
• Model declares a record subclass but column is a string primitive
  → TYPE_MISMATCH.
• RecordSubclass | None ↔ nullable link column happy path.
• An unrelated BaseModel (no id/name) does NOT get treated as
  a link — surfaces as a primitive TYPE_MISMATCH.

Co-Authored-By: Claude noreply@anthropic.com