feat(audio): Campaign A slice A2 — retail's 2D pan+gain mixer replaces AL 3D

Retail is not a 3D audio engine. Every gameplay buffer is created with
m_3D = 0 and the DirectSound 3D listener the client sets up is dead code;
spatialization is two CPU scalars per voice, frozen at emission. This
slice ports that math and demotes OpenAL to a voice bank.

RetailSoundMixer (new, Core) carries the byte-decoded curve from
SoundManager::GetAttenuation @0x00550020: g = dist < 5 ? vol : 25*vol/d2,
clamped to 1 BEFORE the single master multiply, db = ceil(20*log10 g),
with a hard -50 dB floor at which retail does not start the voice at all
(audible radius ~94.2 m at unity). Pan is PlaySoundInternal @0x00550170's
(int)(-15*sin(delta-bearing)) in whole decibels, truncating toward zero,
forced to dead centre when (int)distance < 5, with no front/back and no
elevation cue. Every AL source is now source-relative with rolloff 0 and
the global distance model is None: AL's InverseDistanceClamped was
first-power (2/d), quieter than retail up close and far louder at range
with no cutoff whatsoever. That was the largest audible divergence in the
subsystem (AP-28, retired here).

RetailVoicePool (new, Core) ports the allocator at 0x0054FEC0: ring scan
for a free or finished slot, then evict the first slot whose DAT priority
is strictly lower, else drop. Eviction compared GAIN before, so a loud
unimportant sound could silence a quiet important one. It lives in Core
because the engine's play path talks to native AL handles and could not
be tested; the pool now has 12 conformance tests.

The listener keeps using the camera position, which the decode shows is
retail-faithful (SmartBox::set_viewer @0x00452D36 hands the same collided
camera Position to SoundManager) — only the heading extraction changes,
since retail reads one compass bearing and never a forward/up basis. An
earlier draft of the plan called this a defect; corrected in the plan so
it is not fixed backwards.

Opus review found and this commit fixes: a linear pan-to-azimuth mapping
that saturated to full separation at 30 degrees (OpenAL Soft's own
speaker angle) where retail gives 15 dB — now inverts the constant-power
pan law, so full deflection reaches 0.776 of the arc and both channels
stay live; the stale FUN_00550ad0 / gain-eviction class header, which
contradicted the register row this commit writes; missing discriminating
tests for clamp order and pan truncation; dead PlayingGain state whose
comment invented a retail symbol; and a third in-tree copy of
Position::heading, now delegating to MoveToMath.PositionHeading.

MasterVolume folds into the mixer's one multiply instead of AL listener
gain, so the cutoff, radius and dB quantisation move with the slider.

Register: AP-28 retired; AP-173 (pan law), AP-174 (volume taxonomy),
TS-64 (two unimplemented sound prefs), TS-65 (volume-squared quirk,
applied on the ambient path only) filed. Research note corrected twice
where its summary contradicted its own decode (30 m dB, floor vs trunc).

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

src/AcDream.App/Rendering/WorldRenderFrameBuilder.cs

@@ -7,6 +7,7 @@ using AcDream.App.Rendering.Wb;
 using AcDream.App.Settings;
 using AcDream.App.Streaming;
 using AcDream.App.World;
+using AcDream.Core.Audio;
 using AcDream.Core.Lighting;
 using AcDream.Core.Physics;
 using AcDream.Core.Rendering;
@@ -381,14 +382,19 @@ internal sealed class RuntimeWorldFrameSettingsPreview : IWorldFrameSettingsPrev
         if (_audio is not { IsAvailable: true })
             return;
 
+        // Retail's audio listener IS the camera viewer, refreshed per rendered
+        // frame (`SmartBox::set_viewer` @ 0x00452D36 hands the same collided
+        // camera Position to SoundManager, the sky, and the camera setup), so
+        // the position source here is already faithful. What retail reads off it
+        // is only the origin and `Frame::get_heading` — a compass bearing, not a
+        // basis: pan comes from that heading alone, which is why retail has no
+        // front/back or elevation cue to reproduce.
         Matrix4x4 inverse = camera.InverseView;
         var forward = new Vector3(-inverse.M31, -inverse.M32, -inverse.M33);
-        var up = new Vector3(inverse.M21, inverse.M22, inverse.M23);
         Vector3 position = camera.Position;
         _audio.SetListener(
             position.X, position.Y, position.Z,
-            forward.X, forward.Y, forward.Z,
-            up.X, up.Y, up.Z);
+            RetailSoundMixer.CompassHeadingDegrees(Vector3.Zero, forward));
     }
 }