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/ui/dialogue_box.gd

@@ -140,7 +140,9 @@ func show_dialogue(npc_name: String, speech: String, options: Array = []) -> voi
 	GameState.dialogue_active = true  # D-064: block movement while dialogue visible/fading
 
 	# D-069: Dialogue dip — reduce ambient noise to foreground conversation.
-	# Confrontation options override this with apply_dip("confrontation") in _start_confrontation_beat().
+	# Confrontation options REPLACE (not nest) this dip via apply_dip("confrontation")
+	# in _start_confrontation_beat(). hide_dialogue()'s clear_dip() restores to base
+	# volumes regardless of which profile was last active — intentional replacement semantics.
 	AudioManager.apply_dip("dialogue")
 
 	# D-061: auto-pause in single-player when dialogue opens