feat: FILLET now supports LwPolyline segments and corner filleting

- Add LwPolyline helper functions: nearest-segment finder, segment-to-line
  extractor, corner vertex replacer, segment shortener, bulge calculator
- Add FilletEntity::LwPoly variant carrying poly, handle, and segment index
- compute_fillet_entities handles all new combinations: LwPoly×LwPoly
  (same entity corner fillet with bulge encoding), LwPoly×LwPoly (different
  entities), LwPoly×Line, LwPoly×Arc and their symmetric counterparts
- on_entity_pick detects LwPolyline picks and allows same-entity re-pick for
  adjacent-segment corner filleting
- on_hover_entity preview updated to show LwPolyline wire geometry
- entity_pts extended to cover LwPolyline for preview rendering
- Update prompts to mention LwPolyline as a supported entity type

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

ROADMAP.md

@@ -119,7 +119,7 @@ Underlay (PDF/DWF/DGN)
 | EXTEND (EX) | 🔧 | Spline, LwPolyline |
 | OFFSET (O) | 🔧 | Spline |
 | LENGTHEN (LEN) | 🔧 | Spline, LwPolyline |
-| FILLET (F) | 🔧 | LwPolyline desteği yok |
+| FILLET (F) | ✅ | — |
 | CHAMFER (CHA) | ✅ | — |
 | JOIN (J) | ✅ | — |
 | EXPLODE (X) | ✅ | — |
@@ -303,7 +303,6 @@ Underlay (PDF/DWF/DGN)
 
 ### Yüksek Öncelik
 1. **TRIM / EXTEND / OFFSET / BREAK / LENGTHEN → Spline desteği**
-2. **FILLET → LwPolyline desteği**
 
 ### Orta Öncelik
 5. **Solid3D tessellation** tamamlama (ACIS → truck pipeline)