annotator: bbox tool — create, move, resize, native-pixel coordinates

[JArmandoAnaya]

Context

The primary tool of the 30-minute flow.

Deliverables

• Draw by drag (with min-size threshold to reject accidental clicks), move, resize by
  handles; class assignment from the active class; all through the state machine + command
  log.
• Cursor/handle affordance state exposed to adapters (which handle is hot).

Acceptance criteria

• Scripted event-sequence tests: draw/move/resize produce exact expected geometry
• Undo after each operation restores the prior document