fix(assets): address PR #96 review — all 8 items

- Fix stale 21→18 segment count in architecture doc, script docstrings,
  and disk budget table (critical — both reviewers)
- Add average_m/README.md noting role as clothing reference body
- Add __main__ guards to diagnostic scripts
- Preflight-check all source paths in blender_process_bodies.py
- Document solid-white hair masks as v0.2 placeholder for multi-region
- Add fallback size comment for head mask generation
- Add bald_mask.png (1×1 black) for sidecar convention consistency
- Document child non-uniform scale rationale (Y=0.72 vs XZ=0.75)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

client/assets/characters/bodies/average_m/README.md

@@ -0,0 +1,16 @@
+# average_m — Canonical Clothing Reference Body
+
+`average_m` is the **reference body type** for clothing authoring (D-159).
+
+All clothing items are authored on `average_m` first. The resulting mesh is
+saved as `clothing/<item_id>/reference.glb`. From there, the Surface Deform
+batch pipeline fits it to the remaining 10 body types automatically, producing
+`thin_m.glb`, `thin_f.glb`, `average_f.glb`, etc.
+
+This means:
+- Every clothing item's `reference.glb` was sculpted/modelled for this body
+- `clothing/<item_id>/average_m.glb` is a direct copy of `reference.glb`
+- Do not modify `average_m` body segments without re-running the clothing fit
+
+See `docs/architecture/character-asset-organization.md` Section 5 (Surface Deform
+Batch Output) for the full pipeline.

client/assets/characters/bodies/child/README.md

@@ -5,6 +5,13 @@ a proportional mesh scale to the source body (Teen_Male_FullBody.gltf):
 
   Scale: (0.75, 0.72, 0.75)
 
+The Y scale (0.72) differs from XZ (0.75) intentionally: children have different
+proportions from adults. They are not simply uniformly smaller — their limbs are
+shorter relative to torso height (Y) compared to torso width/depth (XZ). The
+non-uniform scale captures this: the child is 75% of teen width/depth but only
+72% of teen height, reflecting the head-to-body ratio and limb length differences
+typical of pre-teen proportions.
+
 These segments are APPROXIMATE. They produce a visually distinct silhouette
 but are not artist-authored from scratch. Review the proportions and refine
 the base mesh manually if the auto-scale result is not satisfactory.