fix(client): address PR #47 review — murmur deferral, zone extraction, 6 tests

Remove premature Voice/VoiceConversation from SOUND_EVENT_ASSETS (D-072
defers murmur to end-to-end sprint). Extract current_zone_id in
GameState.apply_snapshot() as first-class field, eliminating O(N) tile
scan in main.gd (D-020 server-authoritative). Add dir.list_dir_end()
after registry scan. Add rapid zone-crossing + _load_prefs() roundtrip
tests. Enhance comments on hub/workplace same-asset pattern, station
base hum, and confrontation dip replacement semantics.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

client/scripts/autoloads/audio_manager.gd

@@ -45,7 +45,12 @@ const FILTER_CUTOFF_DEFAULT := 20500.0
 const CROSSFADE_DURATION := 1.8  # D-073: 1.5-2s spec, mid-range
 
 # Maps server zone_id strings to ambient asset keys (filenames in res://audio/).
-# Hub and Workplace share the same ambient layer — same location type.
+# Hub and Workplace intentionally share the same ambient layer (amb_hub_layer) —
+# they are the same location type, so hub→workplace transition is a same-asset no-op
+# (old_asset != new_asset guard skips the fade-out). Sprint brief consolidates
+# D-038's "amb_workplace_layer" to "amb_hub_layer" for v0.1.
+# Note: amb_station_base (D-038 global base hum) plays globally via play_loop()
+# at startup — it is not zone-dependent and has no ZONE_ASSETS entry.
 const ZONE_ASSETS: Dictionary = {
 	"hub":       "amb_hub_layer",
 	"workplace": "amb_hub_layer",
@@ -129,6 +134,7 @@ func _scan_dir(path: String) -> void:
 				if stream:
 					_registry[file_name.get_basename()] = stream
 		file_name = dir.get_next()
+	dir.list_dir_end()
 
 
 func has_asset(asset_key: String) -> bool:
@@ -201,10 +207,6 @@ const SOUND_EVENT_ASSETS: Dictionary = {
 	"FootstepCrouch": "sfx_footstep_metal_walk",  # D-053: same asset until stance-differentiated audio lands
 	"FootstepSprint": "sfx_footstep_metal_run",
 	"FootstepRun":    "sfx_footstep_metal_run",
-	# D-072 (#533): NPC conversation murmur — single universal asset, World SFX bus.
-	# Zone ambient provides natural conspicuousness (bar buries it, corridor exposes it).
-	"Voice":          "sfx_npc_murmur",
-	"VoiceConversation": "sfx_npc_murmur",
 }