Refine the head prototype: flatten face plane, taper jaw, nose/chin relief, fix ear-displacement sign

docs/3d-model-review.md

@@ -217,11 +217,19 @@ head sphere is displaced (not floating spheres) into eye sockets with an orbital
 bridge/tip/ala, upper/lower lip volumes, and extruded ears with a concha recess; the eyeballs
 stay separate. Local renders live under `tmp/npc-parts/npc_casual/head_proto_*.png`.
 
-Vision-judged after two iterations: the nose and ear now protrude in profile (mesh y-min
-−0.186 vs the −0.155 shell front), but the cranium still reads as a "ball with a nose" — the
-face plane is not flattened, the jaw/chin does not taper, and the nose reads as a narrow spike.
-Next refinement before the 1.5 m/1.85 m comparison: flatten the face plane, taper the jaw/chin,
-widen the nose bridge, and shape the back of the skull.
+Iteration log (vision-judged, profile view):
+
+1. Initial displacement too subtle — read as a "ball with props".
+2. Nose + ear visible in profile but the cranium still read as a "ball with a nose".
+3. Added face-plane flatten + jaw/chin taper; taper read, but "~90% sphere".
+4. Stronger flatten + nose + chin; nose now reads, jawline still dissolves into the skull.
+5. Fixed an ear-displacement sign bug (the ± side loop cancelled the helix); the ear now
+   protrudes (x ±0.155 vs the ±0.139 shell) but reads as a soft blob.
+
+Current state (~3/5 human-head read): the nose, mouth notch, chin bump and ear bulge read, but
+three refinements remain before the 1.5 m/1.85 m comparison — a flat forehead with a brow
+step, a defined helix/lobe on the ear, and a jawline that steps back to the neck (the neck is
+part of `_skin_body`, so the standalone head will always lack it).
 
 ## Scorecard skeleton