Add audio subsystem skeleton with a working Linux PCM playback path

Cross-platform audiostream/audio API with per-platform hooks for
PSP/Dolphin/Linux. Linux is fully wired end-to-end through SDL2
(device open, volume-mixed queueing, finish detection via
SDL_GetQueuedAudioSize) and verified playing a generated test tone
in engine.c. PSP and Dolphin hooks are stubbed for now.

src/duskdolphin/audio/audiostreamplatform.h

@@ -0,0 +1,16 @@
+/**
+ * Copyright (c) 2026 Dominic Masters
+ *
+ * This software is released under the MIT License.
+ * https://opensource.org/licenses/MIT
+ */
+
+#pragma once
+#include "audiostreamdolphin.h"
+
+typedef audiostreamdolphin_t audiostreamplatform_t;
+
+#define audioStreamPlatformInit audioStreamDolphinInit
+#define audioStreamPlatformDispose audioStreamDolphinDispose
+#define audioStreamPlatformBuffer audioStreamDolphinBuffer
+#define audioStreamPlatformIsFinished audioStreamDolphinIsFinished