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/settings_dialog.tscn

@@ -0,0 +1,14 @@
+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="Script" path="res://ui/settings_dialog.gd" id="1_settings"]
+
+; #528: Audio settings dialog — 5-bus volume sliders, OPEN_MENU (ESC) to toggle
+[node name="SettingsDialog" type="Control"]
+layout_mode = 3
+anchors_preset = 15
+anchor_right = 1.0
+anchor_bottom = 1.0
+grow_horizontal = 2
+grow_vertical = 2
+mouse_filter = 2
+script = ExtResource("1_settings")