feat: Sketchfab-to-game character pipeline, FP/TP animated views, client-auth netcode, audio system

Character pipeline (tools/):
- sketchfab_import.py: search/download via Sketchfab Download API with
  license/attribution tracking (SKETCHFAB_API_TOKEN)
- autorig.py: headless Blender auto-rig — fits a Mixamo-named skeleton
  to unrigged humanoids, binds automatic weights
- merge_animations.py: merges the shared Mixamo-skeleton animation
  library onto any rigged character, strips root motion, canonical names
- pipeline.py: one command chains download -> rig -> animate -> register

In-game skin system:
- SkinManager autoload reads skins.json (auto-written by the pipeline)
- SkinnedPlayerModel rewritten: canonical clips with fallback chains,
  blend times, speed-scaled locomotion, weapon bone attachment
- First-person: full animated body for the owner, head hidden via
  SkeletonModifier3D; third-person: full model for other players
- Skin selector in main menu; skin id synced in multiplayer
- Fixed GLBLoader crash (GLTFDocument.get_animation_count doesn't exist)

Multiplayer sync overhaul:
- Movement is now client-authoritative: the owning peer simulates
  locally (no input round-trip), server keeps health/kills/death
- Remote players interpolate synced_position/velocity with
  extrapolation and snap-on-teleport
- Knockback/impulses routed to the simulating peer

Audio:
- AudioManager autoload: SFX/Weapons/Footsteps/UI/Music buses, pooled
  3D players, variation + pitch randomization, auto-registration from
  assets/sounds

Docs: 3D_ASSET_PIPELINE.md rewritten end-to-end, new ASSET_SOURCES.md
(non-procedural animation/map/sound sources) and SOUND_DESIGN.md.
Verified with debug/spawn_smoke_test.gd (headless: 24/24 checks pass).

Co-Authored-By: Claude Fable 5 <[email protected]>

weapons/weapon_manager.gd

@@ -313,11 +313,11 @@ func _equip_slot(slot: int) -> void:
 		w.visible = true
 		w.set_process_input(true)
 		
-		# Sync 3rd person weapon
-		if player and player.has_node("HumanoidModel"):
-			var humanoid = player.get_node("HumanoidModel")
-			if humanoid.has_method("set_weapon") and w.has_meta("script_path"):
-				humanoid.set_weapon(w.get_meta("script_path"))
+		# Sync 3rd person weapon on the visual model (skinned or procedural)
+		if player and w.has_meta("script_path"):
+			var visual = player.get_visual_model() if player.has_method("get_visual_model") else player.get_node_or_null("HumanoidModel")
+			if visual and visual.has_method("set_weapon"):
+				visual.set_weapon(w.get_meta("script_path"))
 				# Update the synced variable so remote peers pick it up
 				if "synced_weapon_path" in player:
 					player.synced_weapon_path = w.get_meta("script_path")