feat: material-aware sound occlusion + early reflections for localization

New Acoustics system (globals/acoustics.gd):
- Occlusion: rays from listener to source collect up to 3 occluding bodies;
  each surface's material adds transmission loss and lowers the lowpass
  cutoff. A gunshot through wood stays warm (-8dB, 1.4kHz), brick muffles
  hard (-15dB, 600Hz), concrete harder (-18dB, 450Hz), metal keeps more
  highs (-11dB, 1kHz), glass barely muffles (-5dB, 2.6kHz). Applied via
  each AudioStreamPlayer3D's attenuation filter.
- Early reflections: loud sounds (gunfire, explosions) probe 8 directions
  for nearby walls; each close surface plays a quiet, delayed (path/343ms),
  material-filtered copy from the mirror point — shots audibly slap back
  off structures so players can triangulate. Sub-30ms echoes are skipped
  (they perceptually merge with the direct sound).
- Surface classification: builder-set acoustic_material meta -> node-name
  keywords -> metallic-material inference -> concrete default; cached.

Integration:
- AudioManager.play_3d applies occlusion at fire time and re-filters all
  playing positional sounds every 0.12s (walking behind a wall mid-sound
  audibly muffles it); flight loops opt in via "acoustic_occluded" group
- Remote players' gunshots were entirely SILENT — rpc_play_fire_effects now
  plays the right weapon's shot at the shooter's position, occluded and
  reflected like everything else
- Remote players' footsteps were also silent — now positional + occluded,
  cadence scaled to their speed
- Test level tagged: brick arena walls, wooden platforms/obstacles, metal
  wall-run corridor/rails/dash platforms; dust2 sandstone reads as brick

Tests: debug/acoustics_test.gd (12 checks: per-material loss ordering,
multi-wall stacking, reflection delay/filter, name classification) — all
pass; movement 11/11; smoke 0 failures.

Co-Authored-By: Claude Fable 5 <[email protected]>

weapons/explosive_projectile.gd

@@ -72,6 +72,7 @@ func _setup_effects() -> void:
 	flight_sound.max_db = 0.0
 	flight_sound.max_distance = 45.0
 	flight_sound.volume_db = -8.0
+	flight_sound.add_to_group("acoustic_occluded")  # muffles behind walls
 	add_child(flight_sound)
 	flight_sound.play()
 	

weapons/homing_projectile.gd

@@ -29,6 +29,7 @@ func _start_homing_effects() -> void:
 	flight_sound.max_db = 0.0
 	flight_sound.max_distance = 45.0
 	flight_sound.volume_db = -10.0
+	flight_sound.add_to_group("acoustic_occluded")  # muffles behind walls
 	add_child(flight_sound)
 	flight_sound.play()