feat(audio): Sprint 13 — AudioManager, zone crossfade, dip profiles, murmur wiring (#528, #529, #530, #533)

AudioManager: volume slider persistence (ConfigFile), settings UI with
5 teal-bordered sliders, default_bus_layout.tres for editor. Zone
crossfade: defensive zone_id read from snapshot tiles, 1.5-2s ambient
tween, auto-activates when server ships OQ-09. Dip profiles: dialogue
dip in show/hide_dialogue, ListeningFocus 30-tick gate via
stationary_ticks in game_state.gd. NPC murmur: client plumbing for
event-driven World SFX playback, no-ops until audio asset arrives.

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

client/ui/dialogue_box.gd

@@ -139,6 +139,10 @@ func show_dialogue(npc_name: String, speech: String, options: Array = []) -> voi
 	_is_showing = true
 	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().
+	AudioManager.apply_dip("dialogue")
+
 	# D-061: auto-pause in single-player when dialogue opens
 	SimBridge.send_input({"action": InputMapper.Action.PAUSE, "timestamp_msec": Time.get_ticks_msec()})
 
@@ -156,6 +160,10 @@ func hide_dialogue() -> void:
 	_is_showing = false
 	mouse_filter = Control.MOUSE_FILTER_IGNORE
 
+	# D-069: Clear dialogue/confrontation dip — restore bus volumes to player slider settings.
+	# Safe to call even if confrontation beat already cleared the dip (no-op when empty).
+	AudioManager.clear_dip()
+
 	# D-061: unpause when dialogue closes
 	SimBridge.send_input({"action": InputMapper.Action.UNPAUSE, "timestamp_msec": Time.get_ticks_msec()})