Remove the persistent blood-decal system

Strip only the floor/wall blood-splat decals added in 981ebf1, keeping the
older death-spray, spear-wound spurt, and "Blood & Gore" toggle:

- Delete blood_decals.gd + shader, the Gore autoload (gore.gd), and
  tests/blood_decals_test.gd
- Drop the Gore autoload and the Blood DP section
- Remove all Gore.splat() call sites (matador.gd, bear.gd, player.gd);
  the _blood_burst spray and _spawn_blood spurt stay
- game_shell no longer spawns a per-level decal pool
- Update level_switch test + run_tests.sh accordingly

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CmG3mQGVXZUPDVapmdbWZ1

levels/game_shell.gd

@@ -30,9 +30,6 @@ func load_active_level() -> void:
 	var module := level.level_scene.instantiate()
 	_tag_environment_for_corpses(module)
 	_level_root.add_child(module)
-	# A fresh blood-splat pool for this level, alongside the module under LevelRoot so it's
-	# torn down with everything else on the next load — stains never leak into the next fight.
-	_level_root.add_child(preload("res://blood_decals.gd").new())
 
 
 ## Ground and walls stay on WORLD so the bull and live matadors collide with them; here