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/scenes/main.tscn

@@ -1,4 +1,4 @@
-[gd_scene load_steps=21 format=3 uid="uid://bswrmh7w8dbgm"]
+[gd_scene load_steps=22 format=3 uid="uid://bswrmh7w8dbgm"]
 
 [ext_resource type="Script" path="res://scripts/main.gd" id="1_main"]
 [ext_resource type="Script" path="res://scripts/rendering/world_renderer.gd" id="2_world"]
@@ -20,6 +20,7 @@
 [ext_resource type="PackedScene" path="res://ui/gauntlet_hud.tscn" id="17_gauntlet"]
 [ext_resource type="PackedScene" path="res://ui/checklist_overlay.tscn" id="18_checklist"]
 [ext_resource type="PackedScene" path="res://ui/bug_report_dialog.tscn" id="19_bugreport"]
+[ext_resource type="PackedScene" path="res://ui/settings_dialog.tscn" id="21_settings"]
 
 [node name="Game" type="Node2D"]
 script = ExtResource("1_main")
@@ -162,3 +163,6 @@ layer = 30
 
 ; #495: WRONG button (F12) — bug report capture dialog
 [node name="BugReportDialog" parent="ModalLayer" instance=ExtResource("19_bugreport")]
+
+; #528: Audio settings dialog — 5-bus volume sliders, ESC/OPEN_MENU to toggle
+[node name="SettingsDialog" parent="ModalLayer" instance=ExtResource("21_settings")]