PC3 Phase 1.2: placeholder avatar asset pack + make the loader actually assemble

assets/avatars/catalog.json referenced 14 GLB files that were never in the
repo, so the avatar system could not load a single avatar. And the loader's
assembly path had never run against a real skeleton -- it called
Skeleton3D.has_bone() (no such method in Godot 4) and hand-copied bones in a
way that dropped the skin binding.

- scripts/tools/gen_placeholder_avatars.gd: generates the pack headlessly via
  GLTFDocument -- one rigged/skinned base body (the full 24-bone canonical
  rig from a T-pose) + 13 primitive attachments. Crude on purpose; real art
  swaps in as a pure content change (catalog paths unchanged).
- assets/avatars/*.glb: the generated pack (checked in).
- avatar_loader.gd: rewrote _parse_glb_as_avatar to lift the skin-bound
  Skeleton3D that generate_scene() already produces, instead of rebuilding it.
  Attachments now mount via BoneAttachment3D to the catalog's mount_bone and
  are actually added to the avatar tree (they were parsed then dropped).
  Fixed the has_bone() crash, the null ARRAY_BONES cast in _retarget_skin,
  and the Array[String] type mismatch on the accessory list.
- smoke_test_avatar_system.gd: replaced the "asserts it fails gracefully
  because the files don't exist" section with a real assembly check --
  Skeleton3D present, full 24-bone rig, canonical bones resolve by name,
  visible body mesh, equipped items mounted to bones. 14/14 from a cold cache.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015jtxAiu5dAW4WxCNdGkrkJ

assets/avatars/back_backpack_01.glb.import

@@ -0,0 +1,42 @@
+[remap]
+
+importer="scene"
+importer_version=1
+type="PackedScene"
+uid="uid://dknbtq33pssf4"
+path="res://.godot/imported/back_backpack_01.glb-d0ccc39aed8e86403d649c8d07008591.scn"
+
+[deps]
+
+source_file="res://assets/avatars/back_backpack_01.glb"
+dest_files=["res://.godot/imported/back_backpack_01.glb-d0ccc39aed8e86403d649c8d07008591.scn"]
+
+[params]
+
+nodes/root_type=""
+nodes/root_name=""
+nodes/root_script=null
+nodes/apply_root_scale=true
+nodes/root_scale=1.0
+nodes/import_as_skeleton_bones=false
+nodes/use_name_suffixes=true
+nodes/use_node_type_suffixes=true
+meshes/ensure_tangents=true
+meshes/generate_lods=true
+meshes/create_shadow_meshes=true
+meshes/light_baking=1
+meshes/lightmap_texel_size=0.2
+meshes/force_disable_compression=false
+skins/use_named_skins=true
+animation/import=true
+animation/fps=30
+animation/trimming=false
+animation/remove_immutable_tracks=true
+animation/import_rest_as_RESET=false
+import_script/path=""
+materials/extract=0
+materials/extract_format=0
+materials/extract_path=""
+_subresources={}
+gltf/naming_version=2
+gltf/embedded_image_handling=1