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

run_tests.sh

@@ -26,10 +26,6 @@ echo ""
 echo "=== Touch control tests (joystick self-heals, never strands) ==="
 "$GODOT" --headless --script tests/touch_controls_test.gd
 
-echo ""
-echo "=== Blood decal tests (floor+wall stamps, ring-buffer cap, gore gate) ==="
-"$GODOT" --headless --script tests/blood_decals_test.gd
-
 echo ""
 echo "=== Rigid-skin tests (web mesh conversion keeps all materials) ==="
 "$GODOT" --headless --script tests/rigid_skin_test.gd