feat: Spline support for TRIM, EXTEND, OFFSET, BREAK, and LENGTHEN

Add shared spline_ops module with:
- spline_to_bspline / bspline_to_spline: acadrust ↔ truck BSplineCurve conversion
- spline_sample_xy: sample DXF-XY projection at N+1 points with parameters
- spline_nearest_t: nearest-parameter search via coarse+bisection
- spline_pts_wire: world-space wire points for WireModel preview

TRIM: Spline entities can now be used as cutting boundaries (Geo::Spline
sampled into polyline segments) and as entities being trimmed; uses
BSplineCurve::cut() to produce surviving pieces.

EXTEND: extend_spline() shoots a tangent ray from the nearest endpoint
and appends a control point at the first boundary hit.

OFFSET: offset_spline() offsets each sample point along the local
perpendicular normal and rebuilds a new spline from the offset control pts.

BREAK: break_spline() uses spline_nearest_t() + BSplineCurve::cut() to
split the spline at one or two click points.

LENGTHEN: lengthen_spline() uses numerical arc-length integration +
bisection to cut the spline at the parameter corresponding to the new
requested length.

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

ROADMAP.md

@@ -114,11 +114,11 @@ Underlay (PDF/DWF/DGN)
 | ARRAY Rectangular | ✅ | — |
 | ARRAY Polar | ✅ | — |
 | ARRAY Path | ✅ | — |
-| BREAK (BR) | 🔧 | Spline |
-| TRIM (TR) | 🔧 | Spline |
-| EXTEND (EX) | 🔧 | Spline, LwPolyline |
-| OFFSET (O) | 🔧 | Spline |
-| LENGTHEN (LEN) | 🔧 | Spline, LwPolyline |
+| BREAK (BR) | ✅ | — |
+| TRIM (TR) | ✅ | — |
+| EXTEND (EX) | 🔧 | LwPolyline |
+| OFFSET (O) | ✅ | — |
+| LENGTHEN (LEN) | 🔧 | LwPolyline |
 | FILLET (F) | ✅ | — |
 | CHAMFER (CHA) | ✅ | — |
 | JOIN (J) | ✅ | — |
@@ -302,7 +302,7 @@ Underlay (PDF/DWF/DGN)
 ## Öncelik Sırası (Bir Sonraki Adımlar)
 
 ### Yüksek Öncelik
-1. **TRIM / EXTEND / OFFSET / BREAK / LENGTHEN → Spline desteği**
+1. **EXTEND / LENGTHEN → LwPolyline desteği**
 
 ### Orta Öncelik
 5. **Solid3D tessellation** tamamlama (ACIS → truck pipeline)