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/Composition/SessionPlayerComposition.cs

@@ -10,6 +10,7 @@ using AcDream.App.Rendering.Vfx;
 using AcDream.App.Rendering.Wb;
 using AcDream.App.Runtime;
 using AcDream.App.Settings;
+using AcDream.App.Audio;
 using AcDream.App.Streaming;
 using AcDream.App.Update;
 using AcDream.App.World;
@@ -797,6 +798,44 @@ internal sealed class SessionPlayerCompositionPhase
             new LiveProjectionRescueRebucketter(
                 live.WorldState,
                 live.LiveEntities));
+        // Campaign A slice A5: retail rebuilds the ambient soundscape on every
+        // objcell change (24 m) and drains its deadline queue from the frame
+        // tick. The terrain words come from the loaded landblocks in the 3x3
+        // ring; the listener is the local player's cell and position.
+        IAmbientFramePhase? BuildAmbientFrame()
+        {
+            if (content.Audio?.Ambient is not { } ambient)
+                return null;
+
+            DatReaderWriter.DBObjs.Region? region =
+                content.Dats.Get<DatReaderWriter.DBObjs.Region>(0x13000000u);
+            if (region is null)
+                return null;
+
+            ambient.InstallRegion(region, LoadTerrainWords);
+            return new AmbientFramePhase(
+                ambient,
+                new LocalPlayerAmbientListenerSource(d.PlayerController));
+
+            ushort[]? LoadTerrainWords(uint landblockId)
+            {
+                if (!live.WorldState.TryGetLandblock(landblockId, out LoadedLandblock? loaded)
+                    || loaded?.Heightmap is not { } heightmap)
+                {
+                    return null;
+                }
+
+                // The dat exposes terrain words as TerrainInfo; the gatherer wants
+                // the raw words. Converted here rather than in Core so the ambient
+                // model stays free of the dat type. This runs only on an objcell
+                // change (every 24 m), nine landblocks at a time.
+                var words = new ushort[heightmap.Terrain.Length];
+                for (int i = 0; i < words.Length; i++)
+                    words[i] = (ushort)heightmap.Terrain[i];
+                return words;
+            }
+        }
+
         var liveEffectFrame = new LiveEffectFrameController(
             d.TranslucencyFades,
             content.AnimationHookFrames,
@@ -807,7 +846,8 @@ internal sealed class SessionPlayerCompositionPhase
             content.ParticleSystem,
             content.ScriptRunner,
             d.UpdateClock,
-            new SettingsParticleRangeSource(d.Settings));
+            new SettingsParticleRangeSource(d.Settings),
+            BuildAmbientFrame());
         var liveSpatialReconciler = new LiveSpatialPresentationReconciler(
             live.EntityEffects,
             live.EquippedChildren,