Smooth the NPC neck and shoulder transition

TODO.md

@@ -86,8 +86,11 @@ and equipment fit.
       torso length, hip width, limb length, foot length) and correct the worst outliers.
       (`tools/npc_proportions.py` + `docs/npc-proportion-audit.md`: all eight pass; no skeleton or
       mesh rescue was needed. Head and broad-build silhouettes are nearest their upper bounds.)
-- [ ] Improve neck, shoulder, torso and hip transitions so the silhouette reads as one body rather
-      than stacked primitives; preserve the existing broad/slim/average build variations.
+- [x] Improve the neck and shoulder transition so the silhouette reads as one body rather than a
+      thin post on a hanger. (Fuller neck base/top, lower shoulder root, degenerate-face cleanup;
+      all eight rebuilt and the body comparison sheet is in `docs/blender_previews/`.)
+- [ ] Improve the torso and hip transition; preserve the existing broad/slim/average build
+      variations.
 - [ ] Add subtle body-shape variation between variants beyond uniform scaling (shoulder slope,
       torso taper, hip width, limb thickness) while staying inside the triangle/vertex budget.
 - [ ] Check the procedural fallback and `npc_base` silhouette against the library so the fallback