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>

tooling/blender_process_heads.py

@@ -54,13 +54,19 @@ def process_head(blend_path: str, output_dir: str, head_id: str) -> None:
         print("ERROR: No mesh found in the .blend file")
         sys.exit(1)
 
-    # Detect texture resolution for mask sizing
+    # Detect texture resolution for mask sizing.
+    # NOTE: This picks the first non-HDR texture found in bpy.data.images, which is
+    # fragile — iteration order is not guaranteed and multiple textures may be present.
+    # If no texture is found, falls back to MASK_RESOLUTION (512x512 default) so the
+    # mask is still generated at a sensible size rather than failing.
     mask_w = mask_h = MASK_RESOLUTION
     for img in bpy.data.images:
         if img.size[0] > 0 and img.size[1] > 0 and not img.name.endswith('.hdr'):
             print(f"    Texture found: {img.name} — {img.size[0]}x{img.size[1]}")
             mask_w, mask_h = img.size[0], img.size[1]
             break
+    else:
+        print(f"    No texture found — using fallback mask size {mask_w}x{mask_h}")
 
     # Strip all animation data (heads have no runtime animation — BoneAttachment3D moves them)
     for obj in all_objects: