M7d annotation follow-ups: move-copy deselect + review roll-ups

[Rowee13]

Tracked follow-ups from the M7d (v0.2.4) build and final whole-branch review. All triaged track-not-fix — none are correctness bugs in shipped paths.

1. Move-copy (Ctrl-drag) silently deselects a co-selected annotation

tools/move_tool.py — the move-copy branch calls the pre-existing Selection.replace(instances=new_ids), which zeroes every unnamed field including _annotations. Effect: Ctrl-dragging an instance with an annotation also selected leaves the annotation data untouched (not moved, not copied, not corrupted) but drops it from the selection with no feedback. Move-copy-for-annotations semantics (copy? follow the copy? leave in place?) is a genuine product decision, deliberately out of M7d scope.

2. DeleteAnnotationsCommand.do() uses value-equality removal

commands/annotation_commands.py — uses list.remove(ann) (dataclass __eq__, value-based) rather than the identity-based approach CreateAnnotationCommand uses. Non-exploitable today (ids are model-unique, so no two live annotations are field-for-field equal; candidates are captured by id with true indices before removal). Align to identity-based for consistency.

3. Missing committed test: annotation + stale-instance-id delete

The _on_delete_selection path where an annotation is co-selected with an instance id that no longer exists is verified correct (by trace + adversarial repro) but has no committed regression test.

4. _plan_dimension/_plan_label degenerate degradation is untested at the edges

A degenerate extension line / arrowhead is silently dropped (segment count < expected) rather than raised — consistent with module precedent; graceful (plan still returned, hit boxes intact). Add explicit edge-case coverage if desired.

5. _EPS = 1e-9 is tight for pixel-space vectors

In annotations/draw_plan.py, a near-edge-on view can yield an unstable tick/leader direction (annotation skipped that frame, no NaN; reappears on rotation). Consider a pixel-scaled epsilon.

6. Cosmetic: selected-annotation colour not single-sourced

viewport/viewport_widget.py hardcodes QColor(51, 140, 242) for selected annotations; selected edges use scene_renderer._SELECTION_EDGE_COLOR → (51, 140, 255). Visually indistinguishable; route both through one constant.

Milestone: M7d / v0.2.4. Python-only.