feat(pedit): convert prompt, join, fit/spline/decurve; join fixes (#263)

PEDIT reworked around a mode machine:

- Picking a line/arc asks "Object is not a polyline. Turn it into one?
  [Yes/No] <Y>" — Yes converts in place (arc keeps its bulge) and
  editing continues on the result. Pickfirst: an already-selected
  polyline skips the select step, a selected line/arc goes straight to
  the convert prompt.
- Join gathers with the normal selection system (single picks AND
  window/crossing boxes) and merges through the JOIN machinery.
- Fit replaces every segment with a classic equal-parameter BIARC: two
  arcs per segment meeting tangentially at an inserted knee, both
  segments at a vertex sharing that vertex's tangent — the whole run is
  tangent-continuous (verified: zero tangent jump on L/closed/zigzag/S
  shapes). Spline resamples the vertex frame as a uniform cubic
  B-spline (8 samples per span); Decurve straightens every bulge.
- Earlier hardening kept: undo snapshot before the mutation, immediate
  repaint, polyline-only pick validation.
- Vertex editing lives on grips, so no Edit-vertex submode; instead the
  vertex grip hover menu gains Break — a closed polyline opens at the
  vertex, an open one splits into two (interior vertices only),
  handled driver-side as an entity replacement.

JOIN itself: segs_of now emits one segment per span of an OPEN
polyline (bulges kept), so polylines merge with their neighbours —
PEDIT Join's set always contains the target polyline and used to abort
every time; plain JOIN had the same gap. JOIN also honours pickfirst:
launched with two or more objects selected it joins them immediately.

Closes #263

Co-Authored-By: Claude Fable 5 <[email protected]>

src/scene/model/object.rs

@@ -139,6 +139,9 @@ pub enum GripMenuAction {
     AddVertex,
     RemoveVertex,
     ConvertToArc,
+    /// Split the polyline at this vertex (a closed one opens here; an open
+    /// one splits into two). Handled by the driver — it replaces the entity.
+    BreakVertex,
     ConvertToLine,
     StretchVertex,
     AddLeader,