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 <noreply@anthropic.com>

src/app/update/command.rs

@@ -935,6 +935,33 @@ pub(super) fn on_tab_close(&mut self, idx: usize) -> Task<Message> {
                     self.command_line.push_info(&format!("{label}:"));
                     return self.focus_cmd_input();
                 }
+                // Break at vertex replaces the entity with the split pieces —
+                // structural, so it can't ride the in-place apply below.
+                if matches!(item.action, GripMenuAction::BreakVertex) {
+                    let pieces = match self.tabs[i].scene.document.get_entity(popup.handle) {
+                        Some(acadrust::EntityType::LwPolyline(p)) => {
+                            crate::entities::lwpolyline::break_at_vertex(p, popup.grip_id)
+                        }
+                        _ => None,
+                    };
+                    match pieces {
+                        Some(pieces) => {
+                            self.push_undo_snapshot(i, "BREAK");
+                            self.tabs[i].scene.erase_entities(&[popup.handle]);
+                            for e in pieces {
+                                self.tabs[i].scene.add_entity(e);
+                            }
+                            self.tabs[i].dirty = true;
+                            self.refresh_selected_grips();
+                            self.refresh_properties();
+                            self.command_line.push_output("Polyline broken at vertex.");
+                        }
+                        None => self
+                            .command_line
+                            .push_error("Cannot break at this vertex."),
+                    }
+                    return Task::none();
+                }
                 // One-shot action — apply immediately.
                 self.push_undo_snapshot(i, item.label);
                 // For Add Leader, the new arrow becomes the last grip; remember