feat: third-person camera toggle (V) + fix first-person showing inside the model

The first-person head-shrink (SkeletonModifier3D) left the camera looking at
the inside of the neck/torso/hair. Replaced it: the owner's model now renders
shadows-only in first person (clean FPS view, still fully animated and visible
to others and in shadows).

Added a third-person toggle (V / toggle_camera_view) so you can actually see
your own animated model — the easiest way to verify a new skin's animations:
- over-the-shoulder SpringArm3D camera (wall-collision aware) on the local
  player; press V to swap, press again to return
- firing still uses the first-person camera, so aim is unchanged
- reveals the owner's model via set_owner_visible() on both SkinnedPlayerModel
  and HumanoidModel; hides the first-person weapon viewmodel while in TP
- death forces first person so the toggle can't fight the death cam

Smoke test extended to cover the toggle (camera boom created, becomes current,
round-trips) — 28/28 checks pass for both color and GLB skins.

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

debug/spawn_smoke_test.gd

@@ -87,6 +87,19 @@ func _test_spawn_with_skin(skin_id: String, expect_skinned: bool) -> void:
 				skinned.update_state(state, 9.0, false)
 				await process_frame
 			_check(true, "state cycling did not crash")
+
+	# Third-person toggle: camera swap + owner model visibility.
+	if player.has_method("set_third_person"):
+		var tp_cam = player.get_node_or_null("HeadPivot/ThirdPersonBoom/ThirdPersonCamera")
+		_check(tp_cam != null, "third-person camera boom created")
+		player.set_third_person(true)
+		await process_frame
+		_check(player.third_person, "toggled into third person")
+		if tp_cam:
+			_check(tp_cam.current, "third-person camera is current when toggled on")
+		player.set_third_person(false)
+		await process_frame
+		_check(not player.third_person, "toggled back to first person")
 	else:
 		var humanoid = player.get_node_or_null("HumanoidModel")
 		_check(humanoid != null and humanoid.visible, "procedural model visible for color skin")