feat(audio): Campaign A slice A5 — retail's region ambient soundscape

acdream had no ambient system: StartAmbient minted a handle and played
nothing. Retail's is a weighted-accumulation + timer-queue engine, not
looping voices. On every objcell change (24 m) CellManager::ChangePosition
rebuilds per-sound weights over the 3x3 landblock ring x 64 land cells
each, decoding each cell's terrain word through the region file's
terrain -> scene -> AmbientSTBDesc chain; playback is a min-heap of
absolute deadlines drained from the frame tick, where each pop fires a
one-shot and re-arms.

A continuous bed (base_chance == 0) is non-positional, crossfaded by its
share of the TOTAL weight, and re-fired every min_rate seconds — that
rate is the author's intended loop period, and re-firing is how retail
fakes a sustained bed with no looping voice, re-rolling the variant and
the crossfade each time. An intermittent one keeps its authored volume,
plays at a random accumulated compass bearing at min + (max-min)*t^2,
and is dice-gated. Indoors is silent by design: CEnvCell's contributor is
a folded ret and EnvCell carries no sound data.

The Opus review caught four bugs before this landed, one fatal:

- Cell offsets were built in ABSOLUTE world coordinates and differenced
  against the listener's STREAMED-frame position, so every one of 576
  offsets came out ~32 km, every contribution was culled, and the whole
  feature was silent with nothing logged. Offsets are now landblock-local
  the way Position::get_offset builds them, and the streamed-frame
  position is carried separately for playback, where it belongs.
- The cell's weight was added to the shared denominator once per
  DESCRIPTOR instead of once per CELL, dividing every bed's crossfade by
  the table's entry count — enough to push a typical authored volume
  under the 0.03 audibility floor.
- The drain used  where retail's UseTime is strictly
  below, so a descriptor authored with a zero rate re-armed at the same
  instant and spun the frame forever.
- Arming only enqueued; retail's UpdatePlayQueue PLAYS and then re-arms,
  so a newly audible ambient was silent for a full period after the
  crossing that made it audible.

Also: beds now go through retail's single 16-voice priority pool rather
than acdream's UI pool (retail has one pool; parking beds in the UI pool
let an A4 portal cue chop one mid-wave and discarded the authored
priority), and CalcDir's in-block test is XY-only, since CalcWeight
includes Z on purpose and CalcDir excludes it on purpose.

Two behaviours are knowingly incomplete and registered rather than
guessed at slice end: TS-66 (sky-lit interiors should keep the outdoor
set) and TS-67 (contribution weight is computed in-plane). Retires TS-29.

The frame-loop hook is a typed IAmbientFramePhase, not a callback — the
first attempt used an Action<float> and the architecture guard
ExtractedUpdateOwners_DoNotRetainAnonymousCallbacks correctly rejected it.

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

src/AcDream.App/Update/LiveObjectFrameController.cs

@@ -1,3 +1,4 @@
+using AcDream.App.Audio;
 using System.Numerics;
 using AcDream.App.Input;
 using AcDream.App.Interaction;
@@ -88,7 +89,8 @@ internal sealed class LiveEffectFrameController
         ParticleSystem particles,
         PhysicsScriptRunner scripts,
         IPhysicsScriptTimeSource scriptTime,
-        IParticleRangeSource particleRange)
+        IParticleRangeSource particleRange,
+        IAmbientFramePhase? ambientFrame = null)
     {
         _translucencyFades = translucencyFades
             ?? throw new ArgumentNullException(nameof(translucencyFades));
@@ -102,8 +104,19 @@ internal sealed class LiveEffectFrameController
         _scripts = scripts ?? throw new ArgumentNullException(nameof(scripts));
         _scriptTime = scriptTime ?? throw new ArgumentNullException(nameof(scriptTime));
         _particleRange = particleRange ?? throw new ArgumentNullException(nameof(particleRange));
+        _ambientFrame = ambientFrame;
     }
 
+    /// <summary>
+    /// Campaign A slice A5: retail drains the ambient deadline queue from the
+    /// same per-frame tick that advances the rest of the effect system
+    /// (<c>SmartBox::UseTime</c> -> <c>Ambient::UseTime</c>). A typed
+    /// collaborator rather than a callback — extracted update owners must not
+    /// retain delegates (`ExtractedUpdateOwners_DoNotRetainAnonymousCallbacks`).
+    /// Null when audio is unavailable.
+    /// </summary>
+    private readonly IAmbientFramePhase? _ambientFrame;
+
     public void Tick(float deltaSeconds)
     {
         // Retail's ordinary-object UpdatePositionInternal @ 0x00512C30 calls
@@ -123,6 +136,7 @@ internal sealed class LiveEffectFrameController
         // once-per-host tail and its static-order difference remain TS-51.
         _particles.Tick(deltaSeconds);
         _scripts.Tick(_scriptTime.CurrentScriptTime);
+        _ambientFrame?.TickAmbient(deltaSeconds);
     }
 }