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.Core/Audio/AudioModel.cs

@@ -39,32 +39,12 @@ public sealed class WaveData
     public TimeSpan Duration     { get; init; }
 }
 
-/// <summary>
-/// Falloff math (r05 §2). Retail is CPU-side inverse-square, NOT
-/// DirectSound3DBuffer. No doppler, no cone, no HRTF.
-/// </summary>
-public static class AudioFalloff
-{
-    /// <summary>
-    /// Attenuation factor based on distance. Retail uses pure inverse-square
-    /// above a minimum-distance threshold.
-    /// </summary>
-    public static float AttenuationAt(float distanceMeters, float minDistance = 1.0f)
-    {
-        if (distanceMeters < minDistance) return 1.0f;
-        float att = (minDistance * minDistance) / (distanceMeters * distanceMeters);
-        return Math.Clamp(att, 0f, 1f);
-    }
-
-    /// <summary>
-    /// Stereo pan from listener-relative X coord. ±1.0 fully panned.
-    /// </summary>
-    public static float PanFromRelative(float relativeX, float panRange = 20f)
-    {
-        if (panRange <= 0) return 0f;
-        return Math.Clamp(relativeX / panRange, -1f, 1f);
-    }
-}
+// An `AudioFalloff` helper lived here until 2026-08-08 (Campaign A slice A2).
+// It had the right shape but a 1-metre reference distance where retail's is 5,
+// a `PanFromRelative` that was linear in relative X over an invented 20-metre
+// range where retail pans by the SINE of a compass bearing, and no audibility
+// cutoff. Nothing ever called either method. Both are superseded by
+// `RetailSoundMixer`, which carries the byte-decoded retail math.
 
 /// <summary>
 /// Interface the platform audio engine (AcDream.App layer) implements.
@@ -78,10 +58,17 @@ public interface IAudioEngine : IDisposable
     float MusicVolume  { get; set; }
     float AmbientVolume{ get; set; }
 
-    /// <summary>Update listener pose (called per frame from player position).</summary>
-    void SetListener(float posX, float posY, float posZ,
-                     float forwardX, float forwardY, float forwardZ,
-                     float upX, float upY, float upZ);
+    /// <summary>
+    /// Update the listener pose. Retail's listener is <c>SmartBox::viewer</c> —
+    /// the COLLIDED third-person camera Position, refreshed once per rendered
+    /// frame (<c>SmartBox::update_viewer</c> @ <c>0x00453CE0</c>), falling back
+    /// to the player's own position when the camera sweep fails. Only two
+    /// things are ever read out of it: the origin, for distance, and
+    /// <c>Frame::get_heading</c>, for pan. There is no up vector, no velocity,
+    /// and therefore no doppler and no elevation cue — which is why this takes
+    /// a single compass heading rather than a forward/up basis.
+    /// </summary>
+    void SetListener(float posX, float posY, float posZ, float headingDegrees);
 
     /// <summary>Play a 2D UI sound (no falloff).</summary>
     void PlayUi(SoundId id);