fix #417: world ambience kept playing at character select after the in-world logoff

The character-session reset manifest had no audio step: retail's logoff
destroys the world's sound sources with the world, but our OpenAL world
pool and ambient scheduler are process-lifetime — the continuous ambient
beds played on at character select and the scheduler kept RE-FIRING
deadlines against the stale listener (Suspend/StopAll had zero callers;
WorldGenerationQuiescence only cycles around teleport-style generation
replaces).

New WorldAudioSessionGate: the reset manifest's 'world audio' step stops
all sixteen world-pool voices (SuspendWorldAudio) and drops every ambient
deadline (StopAll); the pool reopens at the entered-world edge through the
new default-null LiveSessionEnteredWorldBindings.ResumeWorldAudio binding,
invoked first in ApplyEnteredWorld. The ambient soundscape needs no
explicit resume — the next objcell observation rebuilds it exactly as a
cell change always did. Covers logout, reconnect, and full stop uniformly.
UI-pool sounds (interface bank, portal cues) untouched by design.

App tests 5568/3 skips, Runtime 1756/0.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.App/Audio/WorldAudioSessionGate.cs

@@ -0,0 +1,39 @@
+namespace AcDream.App.Audio;
+
+/// <summary>
+/// Logout-audio round (2026-08-17): the character-session reset's world-audio
+/// teardown. Retail's logoff destroys every world sound source WITH the world
+/// (<c>CPlayerSystem::ExecuteLogOff @ 0x0055D780</c> world teardown — the
+/// DirectSound buffers die with their owners), so the character-select screen
+/// is silent. acdream's OpenAL voices and the ambient scheduler are
+/// process-lifetime, so the session reset must stop them explicitly:
+/// <see cref="SuspendForSessionReset"/> stops all sixteen world-pool voices
+/// (the continuous ambient beds included) and drops every ambient deadline so
+/// nothing re-fires against the stale listener at character select. The pool
+/// re-opens at the next entered-world edge
+/// (<c>LiveSessionEnteredWorldBindings.ResumeWorldAudio</c>); the ambient
+/// soundscape needs no explicit resume — the next
+/// <c>AmbientSoundController.ObserveListener</c> objcell change rebuilds it,
+/// exactly as a cell change always does.
+/// </summary>
+public sealed class WorldAudioSessionGate
+{
+    private readonly OpenAlAudioEngine _engine;
+    private readonly AmbientSoundController? _ambient;
+
+    public WorldAudioSessionGate(
+        OpenAlAudioEngine engine,
+        AmbientSoundController? ambient)
+    {
+        _engine = engine ?? throw new ArgumentNullException(nameof(engine));
+        _ambient = ambient;
+    }
+
+    public void SuspendForSessionReset()
+    {
+        _engine.SuspendWorldAudio();
+        _ambient?.StopAll();
+    }
+
+    public void ResumeForWorldEntry() => _engine.ResumeWorldAudio();
+}