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/duskpsp/audio/audiostreampsp.h

@@ -0,0 +1,51 @@
+/**
+ * Copyright (c) 2026 Dominic Masters
+ *
+ * This software is released under the MIT License.
+ * https://opensource.org/licenses/MIT
+ */
+
+#pragma once
+#include "error/error.h"
+
+typedef struct audiostream_s audiostream_t;
+
+typedef struct {
+  int channel;
+} audiostreampsp_t;
+
+/**
+ * Initializes the PSP-specific playback state of an audio stream (e.g.
+ * reserving a hardware output channel via sceAudioChReserve).
+ *
+ * @param stream The audio stream to initialize.
+ * @return Error state if any.
+ */
+errorret_t audioStreamPSPInit(audiostream_t *stream);
+
+/**
+ * Disposes the PSP-specific playback state of an audio stream, releasing
+ * its hardware output channel.
+ *
+ * @param stream The audio stream to dispose.
+ * @return Error state if any.
+ */
+errorret_t audioStreamPSPDispose(audiostream_t *stream);
+
+/**
+ * Sends the stream's currently staged PCM data (stream->data) to its
+ * reserved hardware output channel.
+ *
+ * @param stream The audio stream to output.
+ * @return Error state if any.
+ */
+errorret_t audioStreamPSPBuffer(audiostream_t *stream);
+
+/**
+ * Checks whether the stream's reserved hardware channel has finished
+ * playing its currently buffered data.
+ *
+ * @param stream The audio stream to check.
+ * @return true if playback has finished, false otherwise.
+ */
+bool_t audioStreamPSPIsFinished(audiostream_t *stream);