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

matador.gd

@@ -974,7 +974,6 @@ func _enter_ragdoll(hit_dir: Vector3, bull_speed: float, up_boost: float = 0.0)
 	var throw_dir := (hit_dir + Vector3(0.0, 0.5, 0.0)).normalized()
 
 	_blood_burst.burst(global_position + Vector3(0.0, 0.9, 0.0), hit_dir)
-	Gore.splat(global_position, hit_dir, 0.55)
 	if _death_player:
 		_death_player.pitch_scale = randf_range(0.9, 1.1)
 		_death_player.play()