annotator: classification tag tool

[JArmandoAnaya]

Context

Whole-asset classification tags — the third geometry of the beta, and the trivial one.

Deliverables

• Add/remove classification annotations (no coordinates) for classes whose
  geometry is classification_tag (LabelClass.geometry is singular per kernel: SchemaService — monotonic immutable versioning, additive/destructive change classification, LabelClass/Attribute/allowed_geometries validation #6); keyboard-first flow (class hotkey
  toggles the tag).

Acceptance criteria

• Toggle semantics tested; multiple classes can tag the same asset
• Serializes to the ClassificationGeometry wire variant