feat(ui): Audio tab — live volume sliders driving OpenAL engine

Phase L.0 (cont.) — second tab on the Settings shell, in the Easy-wins
build order. Audio is the live-preview poster child: dragging a slider
is audible immediately, Save persists, Cancel reverts and the engine
catches up on the next frame.

AudioSettings record: Master / Music / Sfx / Ambient (all 0..1 floats).
Defaults match the OpenAlAudioEngine constructor values exactly so a
user who never opens the tab gets identical behaviour to the
pre-Phase-L env-var-only world (Master=1.0, Music=0.7, Sfx=1.0,
Ambient=0.8).

SettingsStore grows LoadAudio / SaveAudio + a generic SaveSection
helper that consolidates the unknown-top-level-key preservation logic.
Display and Audio sections coexist in settings.json:
{ "version": 1, "display": { ... }, "audio": { ... } }
Saving one section preserves the other on disk; a future Gameplay /
Chat / Character section drops in the same way without touching
existing data.

SettingsVM gains a parallel audio state machine (audioPersisted /
audioDraft / SetAudio / onSaveAudio callback). HasUnsavedChanges
covers all three buckets now (keybinds + display + audio); Save /
Cancel / ResetAll are atomic across all of them.

GameWindow wiring is the live-preview mechanism — every render frame
pushes the VM's AudioDraft into _audioEngine.MasterVolume etc. Cheap
(four float assignments) and unconditional. SetListener still applies
MasterVolume each frame too via the existing Phase E.2 code path, so
listener gain stays in sync. Persisted audio is applied to the engine
ONCE at startup before the first frame so the user's saved values
take effect before any sound plays — startup-time apply happens during
the same SettingsVM construction site that does the LoadDisplay +
LoadAudio.

SettingsPanel.RenderAudioTab replaces the L.0-shell placeholder — four
SliderFloat calls clamped to [0, 1], plus a footer note explaining the
live-preview UX. The "Coming soon" placeholder test was retargeted
from "Audio" to "Gameplay" since Audio is no longer a placeholder.

16 new tests:
 · AudioSettings record (3) — defaults pin engine constants, value
   equality, with-expressions
 · SettingsStore audio round-trip (5) — missing-file → defaults,
   round-trip all fields, partial-file per-field fallback, save-audio-
   preserves-display, save-display-preserves-audio
 · SettingsVM audio state (5) — initial draft tracks persisted,
   SetAudio marks dirty, Save invokes audio callback, Cancel reverts,
   ResetAllToDefaults covers audio
 · SettingsPanel audio tab (3) — four sliders render only when active,
   no SliderFloat emitted on inactive tabs, slider range is [0, 1]

dotnet build green (0 warnings); dotnet test 1,262 / 1,262 green
(243 Core.Net + 346 UI.Abstractions + 673 Core).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/AcDream.UI.Abstractions.Tests/Panels/Settings/SettingsStoreTests.cs

@@ -116,4 +116,68 @@ public sealed class SettingsStoreTests : System.IDisposable
         var path = SettingsStore.DefaultPath();
         Assert.EndsWith("acdream" + Path.DirectorySeparatorChar + "settings.json", path);
     }
+
+    // -- Audio section round-trip ----------------------------------------
+
+    [Fact]
+    public void LoadAudio_returns_defaults_when_file_is_missing()
+    {
+        var store = new SettingsStore(_tempPath);
+        Assert.Equal(AudioSettings.Default, store.LoadAudio());
+    }
+
+    [Fact]
+    public void SaveAudio_then_LoadAudio_round_trips_all_fields()
+    {
+        var store = new SettingsStore(_tempPath);
+        var original = new AudioSettings(Master: 0.3f, Music: 0.45f, Sfx: 0.9f, Ambient: 0.6f);
+
+        store.SaveAudio(original);
+        var loaded = store.LoadAudio();
+
+        Assert.Equal(original, loaded);
+    }
+
+    [Fact]
+    public void LoadAudio_falls_back_per_field_when_keys_missing()
+    {
+        File.WriteAllText(_tempPath, """
+            {
+              "version": 1,
+              "audio": { "master": 0.25 }
+            }
+            """);
+        var store = new SettingsStore(_tempPath);
+
+        var loaded = store.LoadAudio();
+
+        Assert.Equal(0.25f, loaded.Master);
+        Assert.Equal(AudioSettings.Default.Music, loaded.Music);
+        Assert.Equal(AudioSettings.Default.Sfx, loaded.Sfx);
+        Assert.Equal(AudioSettings.Default.Ambient, loaded.Ambient);
+    }
+
+    [Fact]
+    public void SaveAudio_preserves_display_section()
+    {
+        // Save display first, then audio — display values must survive.
+        var store = new SettingsStore(_tempPath);
+        store.SaveDisplay(DisplaySettings.Default with { Resolution = "2560x1440" });
+        store.SaveAudio(AudioSettings.Default with { Master = 0.4f });
+
+        Assert.Equal("2560x1440", store.LoadDisplay().Resolution);
+        Assert.Equal(0.4f, store.LoadAudio().Master);
+    }
+
+    [Fact]
+    public void SaveDisplay_after_SaveAudio_preserves_audio_section()
+    {
+        // Reverse order — audio must survive a subsequent display save.
+        var store = new SettingsStore(_tempPath);
+        store.SaveAudio(AudioSettings.Default with { Music = 0.1f });
+        store.SaveDisplay(DisplaySettings.Default with { ShowFps = true });
+
+        Assert.Equal(0.1f, store.LoadAudio().Music);
+        Assert.True(store.LoadDisplay().ShowFps);
+    }
 }