Fix blaster GLB root type error (Node3D, not MeshInstance3D)

- Changed _gun_model type from MeshInstance3D to Node3D
  (all imported GLB/GLTF weapon models have Node3D root)
- Added check_models.gd diagnostic script

The type mismatch at WeaponManager._load_weapon_models:73
was the only script error in the user's game.log.
Headless bot tests confirm movement still works fine.

examples/multiplayer-fps/scripts/weapon_manager.gd

@@ -26,7 +26,7 @@ var _pending_grenades: Array[Dictionary] = []  # Buffered throws, consumed in _a
 
 # Weapon models (loaded at runtime)
 var _knife_model: Node3D
-var _gun_model: MeshInstance3D
+var _gun_model: Node3D
 var _grenade_model: Node3D
 var _current_visible_slot: int = -1
 var _knife_sound: AudioStreamPlayer3D

tests/check_models.gd

@@ -0,0 +1,20 @@
+#!/usr/bin/env -S godot --headless --script
+extends SceneTree
+
+func _init():
+	var paths = [
+		"res://assets/kenney/models/blaster.glb",
+		"res://examples/multiplayer-fps/models/knife.fbx",
+		"res://examples/multiplayer-fps/models/grenade.glb",
+	]
+	for p in paths:
+		var scene = load(p)
+		if scene:
+			var inst = scene.instantiate()
+			print(p, " -> type: ", inst.get_class(), " children: ", inst.get_child_count())
+			for c in inst.get_children():
+				print("   child: ", c.get_class(), " ", c.name)
+			inst.queue_free()
+		else:
+			print(p, " -> FAILED TO LOAD")
+	quit()

tests/check_models.gd.uid

@@ -0,0 +1 @@
+uid://c2m50xd561do8