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

player.gd

@@ -1205,9 +1205,6 @@ func take_sword_hit(cause: String = "", hit_pos: Vector3 = Vector3.ZERO) -> void
 	# bull). Melee gores pass no position and skip the spurt; gore can be turned off.
 	if cause == "thrown" and Settings.gore:
 		_spawn_blood(hit_pos if hit_pos != Vector3.ZERO else global_position)
-	# Any hit — melee or thrown — leaves a lasting stain on the ground the bull bled on.
-	var splat_dir := global_position - hit_pos if hit_pos != Vector3.ZERO else Vector3.ZERO
-	Gore.splat(global_position, splat_dir, 0.5)
 	if _huff_player:
 		_huff_player.pitch_scale = randf_range(0.7, 0.9)
 		_huff_player.play()