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]>

debug/dust2_level_builder.gd

@@ -42,9 +42,10 @@ func _build_dust2_layout() -> void:
 	var floor_mat = LevelMaterials.tinted(Color(0.7, 0.65, 0.55))  # Dusty ground
 	var box_mat = LevelMaterials.tinted(Color(0.4, 0.3, 0.2), true)  # Wood crates
 	
-	# Root CSG
+	# Root CSG — sandstone: the whole map reads acoustically as brick
 	var root_csg = CSGCombiner3D.new()
 	root_csg.use_collision = true
+	root_csg.set_meta("acoustic_material", "brick")
 	add_child(root_csg)
 	
 	# Huge solid block representing the world bounds