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/command_driver.rs

@@ -1277,25 +1277,58 @@ impl OpenCADStudio {
                 self.restore_pre_cmd_tangent();
             }
             CmdResult::PeditOp { handle, op } => {
-                use crate::modules::draw::modify::pedit::apply_pedit;
-                let changed = self.tabs[i]
-                    .scene
-                    .document
-                    .get_entity_mut(handle)
-                    .map(|e| apply_pedit(e, &op))
-                    .unwrap_or(false);
-                if changed {
-                    self.push_undo_snapshot(i, "PEDIT");
-                    self.tabs[i].dirty = true;
-                    self.command_line.push_output("PEDIT: applied.");
-                    self.refresh_properties();
-                } else {
-                    self.command_line
-                        .push_error("PEDIT: operation not applicable to this entity.");
+                use crate::modules::draw::modify::pedit::{
+                    apply_pedit, convert_to_polyline, PeditOp,
+                };
+                match &op {
+                    // The convert replaces the entity (new handle).
+                    PeditOp::ConvertToPolyline => {
+                        let converted = self.tabs[i]
+                            .scene
+                            .document
+                            .get_entity(handle)
+                            .and_then(convert_to_polyline);
+                        match converted {
+                            Some(pl) => {
+                                return self.apply_cmd_result(CmdResult::ReplaceEntity(
+                                    handle,
+                                    vec![pl],
+                                ));
+                            }
+                            None => self
+                                .command_line
+                                .push_error("PEDIT: cannot convert this entity."),
+                        }
+                    }
+                    _ => {
+                        // Snapshot BEFORE the mutation — an after-the-fact
+                        // snapshot records the changed state and undo no-ops.
+                        self.push_undo_snapshot(i, "PEDIT");
+                        let changed = self.tabs[i]
+                            .scene
+                            .document
+                            .get_entity_mut(handle)
+                            .map(|e| apply_pedit(e, &op))
+                            .unwrap_or(false);
+                        if changed {
+                            self.tabs[i].dirty = true;
+                            // Repaint immediately (wide-band fill included).
+                            self.tabs[i].scene.mark_entity_dirty(handle);
+                            self.tabs[i].scene.refresh_fill_model(handle);
+                            self.tabs[i].scene.bump_geometry_no_blocks();
+                            self.command_line.push_output("PEDIT: applied.");
+                            self.refresh_properties();
+                        } else {
+                            let _ = self.tabs[i].history.undo_stack.pop();
+                            self.command_line
+                                .push_error("PEDIT: operation not applicable to this entity.");
+                        }
+                    }
+                }
+                let prompt = self.tabs[i].active_cmd.as_ref().map(|c| c.prompt());
+                if let Some(p) = prompt {
+                    self.command_line.push_info(&p);
                 }
-                // Keep command active — user may apply more ops
-                self.command_line
-                    .push_info("PEDIT  Enter option [C=Close O=Open W=Width X=Exit]:");
             }
             CmdResult::JoinEntities(handles) => {
                 use crate::modules::draw::modify::join::join_entities;