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]>

debug/test_level_builder.gd

@@ -299,15 +299,9 @@ func _spawn_player(pid: int) -> CharacterBody3D:
 	server_sync.name = "ServerSynchronizer"
 	server_sync.set_multiplayer_authority(1) # Host always controls these
 	var server_rep_config = SceneReplicationConfig.new()
-	server_rep_config.add_property(":position")
-	server_rep_config.add_property(":synced_movement_state")
-	server_rep_config.add_property(":synced_movement_speed")
-	server_rep_config.add_property(":synced_is_crouching")
 	server_rep_config.add_property(":health")
 	server_rep_config.add_property(":shield")
 	server_rep_config.add_property(":is_dead")
-	server_rep_config.add_property(":synced_grapple_point")
-	server_rep_config.add_property(":synced_is_grapple_shooting")
 	server_sync.replication_config = server_rep_config
 	player.add_child(server_sync)
 
@@ -316,8 +310,16 @@ func _spawn_player(pid: int) -> CharacterBody3D:
 	client_sync.name = "MultiplayerSynchronizer" # Keep original name for compatibility if needed elsewhere
 	client_sync.set_multiplayer_authority(pid)
 	var client_rep_config = SceneReplicationConfig.new()
+	client_rep_config.add_property(":synced_position")
+	client_rep_config.add_property(":synced_velocity")
 	client_rep_config.add_property(":rotation")
 	client_rep_config.add_property("HeadPivot:rotation")
+	client_rep_config.add_property(":synced_movement_state")
+	client_rep_config.add_property(":synced_movement_speed")
+	client_rep_config.add_property(":synced_is_crouching")
+	client_rep_config.add_property(":synced_grapple_point")
+	client_rep_config.add_property(":synced_is_grapple_shooting")
+	client_rep_config.add_property(":synced_skin_id")
 	client_rep_config.add_property(":synced_weapon_path")
 	client_rep_config.add_property(":synced_loadout_p1")
 	client_rep_config.add_property(":synced_loadout_p2")