feat: SPLINEDIT — close, open, reverse spline control-point editing

- splinedit.rs: SplineditCommand picks a spline (entity-pick phase) then
  accepts CLOSE/OPEN/REVERSE/EXIT sub-commands via text input
- apply_spline_op() mutates the spline in the document directly, with knot
  vector regeneration via generate_clamped_knots()
- Sentinel XLine trick: ReplaceEntity with a magic __SPLINEDIT_* layer name
  is intercepted in cmd_result.rs before erasing the original entity
- Control-point dragging already works via the existing grip editing system
- ROADMAP.md: SPLINEDIT marked ✅

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/commands.rs

@@ -1435,6 +1435,13 @@ impl H7CAD {
                 self.tabs[i].active_cmd = Some(Box::new(cmd_obj));
             }
 
+            "SPLINEDIT"|"SPE" => {
+                use crate::modules::home::modify::splinedit::SplineditCommand;
+                let cmd_obj = SplineditCommand::new();
+                self.command_line.push_info(&cmd_obj.prompt());
+                self.tabs[i].active_cmd = Some(Box::new(cmd_obj));
+            }
+
             "ALIGN"|"AL" => {
                 use crate::modules::home::modify::align::AlignCommand;
                 let cmd = AlignCommand::new();