annotator: document model — UUID-addressed annotations, stable selection, schema as typed input

[JArmandoAnaya]

Context

The engine's document: annotations, selection, active tool state — with identity semantics
matching the kernel wire format.

Deliverables

• Document type: annotations keyed by UUID (id generated client-side, uuid v4), asset
  descriptor (dimensions), schema (classes with colors + one geometry each — LabelClass.geometry is singular per kernel: SchemaService — monotonic immutable versioning, additive/destructive change classification, LabelClass/Attribute/allowed_geometries validation #6) as typed
  input.
• Selection model: by UUID set; survives reordering, undo/redo, and annotation edits.
• Serialization to/from the kernel wire shapes (matching the pydantic discriminated union
  from M1) — the boundary where the generated API types plug in at M5.

Acceptance criteria

• Selection stable across undo/redo in tests
• Round-trip with wire-format fixtures (bbox/polygon/classification)