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/Composition/SessionPlayerComposition.cs

@@ -1118,6 +1118,11 @@ internal sealed class SessionPlayerCompositionPhase
                 live.SelectionInteractions),
             new LiveSessionWorldRuntime(
                 content.Dats,
+                content.Audio?.Engine is { } sessionAudioEngine
+                    ? new AcDream.App.Audio.WorldAudioSessionGate(
+                        sessionAudioEngine,
+                        content.Audio.Ambient)
+                    : null,
                 live.WorldState,
                 live.LiveEntities,
                 sessionEvents,