Add the hair part library and give the roster three silhouettes

docs/blender-assets.md

@@ -176,6 +176,12 @@ colour texture. The current `_apply_outfit_materials()` sets
 `albedo_color = WHITE` on any slot it does not recognise, and these keep their
 colour through that. When integrating, tint only the five contract slots.
 
+**Hair part library.** Each base ships one hair shell and the build reshapes it in place
+(`QUATERNIUS_HAIR_PARTS`): `short` is identity, `buzz` squashes toward the hairline, `long` stretches
+from the crown. The role picks the style through `CHARACTERS[...]["hair"]`, and the T-pose cache is
+keyed on it. `bald` is only allowed on the `casual` and `hoodie` bases, where the skin closes the
+skull; the modular Worker's head stops at the hairline, so removing its hair would open the crown.
+
 **Face detail.** None of the three Quaternius heads ships a mouth or eye whites, so the build adds
 both: `_add_quaternius_mouth()` places a lip block on the measured skin surface, and
 `_add_quaternius_eye_whites()` renames the eye quads to `sclera` and adds a shrunken `pupil` copy in