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/default_bus_layout.tres

@@ -0,0 +1,43 @@
+[gd_resource type="AudioBusLayout" format=3]
+
+; D-068: 5-bus audio architecture — Music, Ambient, WorldSFX, PlayerActions, UISounds.
+; All buses route to Master. Volumes managed at runtime by AudioManager autoload.
+; AudioManager._setup_buses() creates any missing buses on startup (no-op if present).
+
+[resource]
+bus/0/name = "Master"
+bus/0/solo = false
+bus/0/mute = false
+bus/0/bypass_fx = false
+bus/0/volume_db = 0.0
+bus/0/send = &""
+bus/1/name = "Music"
+bus/1/solo = false
+bus/1/mute = false
+bus/1/bypass_fx = false
+bus/1/volume_db = 0.0
+bus/1/send = &"Master"
+bus/2/name = "Ambient"
+bus/2/solo = false
+bus/2/mute = false
+bus/2/bypass_fx = false
+bus/2/volume_db = 0.0
+bus/2/send = &"Master"
+bus/3/name = "WorldSFX"
+bus/3/solo = false
+bus/3/mute = false
+bus/3/bypass_fx = false
+bus/3/volume_db = 0.0
+bus/3/send = &"Master"
+bus/4/name = "PlayerActions"
+bus/4/solo = false
+bus/4/mute = false
+bus/4/bypass_fx = false
+bus/4/volume_db = 0.0
+bus/4/send = &"Master"
+bus/5/name = "UISounds"
+bus/5/solo = false
+bus/5/mute = false
+bus/5/bypass_fx = false
+bus/5/volume_db = 0.0
+bus/5/send = &"Master"