annotator: 2D geometry — hit-testing, handles, clamp to asset bounds, transforms (port of v1's ~220 LOC of pure functions)

[JArmandoAnaya]

Context

v1's geometry is ~220 LOC of pure, battle-tested functions. Port it, don't rewrite it.

Deliverables

• Hit-testing: point-in-bbox, point-in-polygon, handle/vertex/edge proximity with
  zoom-aware tolerances.
• Bbox transforms: move, resize by 8 handles with min-size + bounds clamping; polygon:
  vertex move, edge insert point, vertex delete.
• All coordinates in native asset pixels (screen↔image transforms live at the adapter
  layer, not in core geometry).

Acceptance criteria

• Ported functions covered by unit tests (bring v1 cases where they exist)
• Clamping property test: transforms never escape asset bounds