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

bear.gd

@@ -884,7 +884,6 @@ func _take_hit(hit_dir: Vector3, strength: float) -> void:
 	hit_dir.y = 0.0
 	hit_dir = hit_dir.normalized()
 	_blood_burst.burst(global_position + Vector3(0.0, 1.4, 0.0), hit_dir)
-	Gore.splat(global_position, hit_dir, 1.1)
 	_shake(clampf(strength / 15.0, 0.4, 1.0))
 	if _hp <= 0:
 		_die(hit_dir)
@@ -920,7 +919,6 @@ func _die(hit_dir: Vector3) -> void:
 		_hit_area.monitoring = false
 	velocity = Vector3.ZERO
 	_blood_burst.burst(global_position + Vector3(0.0, 1.4, 0.0), hit_dir)
-	Gore.splat(global_position, hit_dir, 1.4)
 	_play(_anim_death)
 	# No death clip on the rig — topple the beast onto its side (about the hit direction,
 	# so it falls the way it was struck) and sink it away, then free.