feat(audio): Ctrl+M instant mute + inn-chatter investigation closed as server content

Two items from listening-gate round 2.

Mute: AcdreamToggleAudioMute (default Ctrl+M; bare M is selection) flips
OpenAlAudioEngine.Muted, implemented as the AL LISTENER gain - unused
since A2 moved all mixing to the CPU, so it is a free master switch that
silences already-playing voices instantly and restores them exactly,
without touching the retail mixing math, the -50 dB allocation cutoff, or
any persisted volume setting. Rebindable like every other action; console
line confirms each flip.

Inn chatter: the user hears talk-and-laughter ambience in retail inns and
not in acdream. Three installed-dat scans (pinned as conformance tests in
EnvCellSoundEmitterInventoryTests) prove the mechanism is NOT client
data: no interior static in the town landblock carries an ambient-slot
sound table, no Setup among all 5,935 in the portal dat references one,
and yet 23 sound tables carrying ONLY Ambient1..8 slots exist - pure
soundscape banks with nothing client-side pointing at them. They are
wire-bound: the server attaches one to an emitter object via
CreateObject's sound-table field and fires the slots over 0xF750 - ACE
implements exactly this (EmoteType.Sound heartbeat emotes ->
GameMessageSound broadcast). Our 0xF750 receiver (slice A3) is live and
now instrumented (ACDREAM_PROBE_SOUND_WIRE=1, via the new
AudioDiagnostics owner per Code Structure Rule 5, with per-event drop
reasons in AudioHookSink.PlayServerSound). A probed session against the
local ACE received ZERO 0xF750 events across a town walkabout: the
silence is server world-content (no emitters configured/firing), not a
client drop. The first scan's assertion originally encoded the
emitter-object hypothesis; the data refuted it, and the test now pins the
negative so the conclusion cannot silently rot.

Full Release suite green (the one failure during development was the
hypothesis-pinning assertion, corrected to pin the finding).

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

src/AcDream.Core/Audio/AudioDiagnostics.cs

@@ -0,0 +1,22 @@
+using System;
+
+namespace AcDream.Core.Audio;
+
+/// <summary>
+/// Diagnostic toggles for the audio subsystem, read once at startup per the
+/// Code Structure Rules (one static owner per subsystem; no per-call-site
+/// env reads).
+/// </summary>
+public static class AudioDiagnostics
+{
+    /// <summary>
+    /// One console line per inbound server Sound event (0xF750) and per
+    /// wire-sound play decision, with the drop reason when nothing plays.
+    /// Used to pin whether interior soundscapes (inn chatter — server-bound
+    /// ambient sound tables fired by emitter heartbeats) are absent because
+    /// the server never sends them or because the client drops them.
+    /// <para>Initial state from <c>ACDREAM_PROBE_SOUND_WIRE=1</c>.</para>
+    /// </summary>
+    public static bool ProbeWireSoundsEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_SOUND_WIRE") == "1";
+}