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/commands/inquiry.rs

@@ -223,6 +223,15 @@ impl OpenCADStudio {
             // ── Break / Join ─────────────────────────────────────────────────
             "JOIN" => {
                 use crate::modules::draw::modify::join::JoinCommand;
+                // Pickfirst: with objects already selected, join them right
+                // away instead of asking for a selection again.
+                let selected: Vec<acadrust::Handle> =
+                    self.tabs[i].scene.selected.iter().copied().collect();
+                if selected.len() >= 2 {
+                    let task =
+                        self.apply_cmd_result(crate::command::CmdResult::JoinEntities(selected));
+                    return Some(task);
+                }
                 let cmd = JoinCommand::new();
                 self.command_line.push_info(&cmd.prompt());
                 self.tabs[i].active_cmd = Some(Box::new(cmd));
@@ -243,8 +252,40 @@ impl OpenCADStudio {
             }
 
             "PEDIT" => {
-                use crate::modules::draw::modify::pedit::PeditCommand;
-                let cmd_obj = PeditCommand::new();
+                use crate::modules::draw::modify::pedit::{PeditCommand, PeditTarget};
+                let info = self.tabs[i]
+                    .scene
+                    .document
+                    .entities()
+                    .filter_map(|e| {
+                        let h = e.common().handle.value();
+                        match e {
+                            acadrust::EntityType::LwPolyline(_)
+                            | acadrust::EntityType::Polyline2D(_) => Some((
+                                h,
+                                PeditTarget {
+                                    is_poly: true,
+                                    convertible: false,
+                                },
+                            )),
+                            acadrust::EntityType::Line(_) | acadrust::EntityType::Arc(_) => {
+                                Some((
+                                    h,
+                                    PeditTarget {
+                                        is_poly: false,
+                                        convertible: true,
+                                    },
+                                ))
+                            }
+                            _ => None,
+                        }
+                    })
+                    .collect();
+                // Pickfirst: an already-selected polyline (or line/arc, via
+                // the convert prompt) skips the select step.
+                let preselected: Vec<acadrust::Handle> =
+                    self.tabs[i].scene.selected.iter().copied().collect();
+                let cmd_obj = PeditCommand::new(info).with_preselection(&preselected);
                 self.command_line.push_info(&cmd_obj.prompt());
                 self.tabs[i].active_cmd = Some(Box::new(cmd_obj));
             }