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>

ROADMAP.md

@@ -125,7 +125,7 @@ Underlay (PDF/DWF/DGN)
 | EXPLODE (X) | ✅ | — |
 | PEDIT (PE) | ✅ | — |
 | STRETCH (SS) | ✅ | — |
-| SPLINEDIT | ⬜ | — |
+| SPLINEDIT | ✅ CLOSE/OPEN/REVERSE + grip editing | — |
 | HATCHEDIT | ✅ | — |
 | ATTEDIT | ⬜ | Attribute değerlerini düzenleme |
 | DDEDIT (çift tık metin) | ✅ | — |