feat(pline): Undo option removes the last segment (#352)

U / UNDO (typed or via the prompt button) pops the last placed vertex
without ending the command, repeatably down to and past the start
point. The exit tangent of the remaining last segment is recomputed
from its bulge so a following Arc segment stays tangent-continuous.
New CmdResult::RemoveLiveEntity takes the live polyline out of the
document when Undo drops it below two vertices; the next point
re-creates it under the same undo unit.

Closes #352

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/app/command_driver.rs

@@ -808,6 +808,19 @@ impl OpenCADStudio {
                     }
                 }
             }
+            CmdResult::RemoveLiveEntity(handle) => {
+                // The command backed off below a valid entity (PLINE Undo at
+                // one remaining vertex): take the live entity out of the
+                // document and keep prompting. No undo bookkeeping — the
+                // create's snapshot already covers the whole in-progress
+                // object as one unit.
+                self.tabs[i].scene.erase_entities(&[handle]);
+                self.tabs[i].dirty = true;
+                let prompt = self.tabs[i].active_cmd.as_ref().map(|c| c.prompt());
+                if let Some(p) = prompt {
+                    self.command_line.push_info(&p);
+                }
+            }
             CmdResult::Cancel => {
                 self.tabs[i].scene.clear_preview_wire();
                 self.tabs[i].active_cmd = None;