Add anchor gizmo dots, keymap click handler, and auto-sync for parametric dimensions

- GizmoAnchorHandle + DimensionAnchorWidget: colored dot gizmos at each
  dimension curve vertex (green=anchored, orange=free); color changes to
  blue while SetDimensionAnchor is in PICK_FACE mode for that vertex
- ClickNearestDimensionAnchor (LMB keymap): Python proximity operator that
  fires SetDimensionAnchor pre-targeted at the nearest anchor dot within
  120px, returning PASS_THROUGH for misses so normal viewport clicks are
  unaffected
- SetDimensionAnchor: added anchor_index prop to enter PICK_FACE directly;
  set_active_anchor called at all phase transitions (invoke, vertex-pick,
  face-pick, alt-click free, ESC/RMB) so gizmo color tracks state correctly
- handler._sync_dimension_anchors_to_curve: proximity-based anchor sync
  when curve vertex count changes in Edit Mode (subdivide / delete)
- depsgraph_update_post_handler: regenerates dimensions when referenced
  elements move; also handles annotation curve edits directly
- Remove standalone Set Anchor button from annotation tool UI (replaced by
  clicking a gizmo dot)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/bonsai/bonsai/bim/module/drawing/workspace.py

@@ -262,7 +262,6 @@ class AnnotationToolUI:
                     row.prop(ann_props, "line_position")
                 cls.layout.separator()
                 row = cls.layout.row(align=True)
-                row.operator("bim.set_dimension_anchor", icon="PIVOT_CURSOR")
                 op = row.operator("bim.regenerate_dimensions", icon="FILE_REFRESH", text="Regenerate")
                 op.active_only = True