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/audiodolphin.c

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+/**
+ * Copyright (c) 2026 Dominic Masters
+ *
+ * This software is released under the MIT License.
+ * https://opensource.org/licenses/MIT
+ */
+
+#include "audiodolphin.h"
+
+errorret_t audioDolphinInit() {
+  errorOk();
+}
+
+errorret_t audioDolphinUpdate() {
+  errorOk();
+}
+
+errorret_t audioDolphinDispose() {
+  errorOk();
+}

src/duskdolphin/audio/audiodolphin.h

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+/**
+ * Copyright (c) 2026 Dominic Masters
+ *
+ * This software is released under the MIT License.
+ * https://opensource.org/licenses/MIT
+ */
+
+#pragma once
+#include "error/error.h"
+
+/**
+ * Initializes the GameCube/Wii-specific audio subsystem state.
+ *
+ * @return Error state if any.
+ */
+errorret_t audioDolphinInit();
+
+/**
+ * Updates the GameCube/Wii-specific audio subsystem state. Called once per
+ * frame.
+ *
+ * @return Error state if any.
+ */
+errorret_t audioDolphinUpdate();
+
+/**
+ * Disposes the GameCube/Wii-specific audio subsystem state.
+ *
+ * @return Error state if any.
+ */
+errorret_t audioDolphinDispose();

src/duskdolphin/audio/audioplatform.h

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+/**
+ * Copyright (c) 2026 Dominic Masters
+ *
+ * This software is released under the MIT License.
+ * https://opensource.org/licenses/MIT
+ */
+
+#pragma once
+#include "audiodolphin.h"
+
+#define audioPlatformInit audioDolphinInit
+#define audioPlatformUpdate audioDolphinUpdate
+#define audioPlatformDispose audioDolphinDispose

src/duskdolphin/audio/audiostreamdolphin.c

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+/**
+ * Copyright (c) 2026 Dominic Masters
+ *
+ * This software is released under the MIT License.
+ * https://opensource.org/licenses/MIT
+ */
+
+#include "audiostreamdolphin.h"

src/duskdolphin/audio/audiostreamdolphin.h

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+/**
+ * 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 {
+  int32_t voiceId;
+} audiostreamdolphin_t;
+
+/**
+ * Initializes the GameCube/Wii-specific playback state of an audio stream
+ * (e.g. allocating a voice via AESND_AllocateVoice/ansnd equivalent).
+ *
+ * @param stream The audio stream to initialize.
+ * @return Error state if any.
+ */
+errorret_t audioStreamDolphinInit(audiostream_t *stream);
+
+/**
+ * Disposes the GameCube/Wii-specific playback state of an audio stream,
+ * releasing its voice.
+ *
+ * @param stream The audio stream to dispose.
+ * @return Error state if any.
+ */
+errorret_t audioStreamDolphinDispose(audiostream_t *stream);
+
+/**
+ * Sends the stream's currently staged PCM data (stream->data) to its
+ * allocated voice.
+ *
+ * @param stream The audio stream to output.
+ * @return Error state if any.
+ */
+errorret_t audioStreamDolphinBuffer(audiostream_t *stream);
+
+/**
+ * Checks whether the stream's voice has finished playing its currently
+ * buffered data.
+ *
+ * @param stream The audio stream to check.
+ * @return true if playback has finished, false otherwise.
+ */
+bool_t audioStreamDolphinIsFinished(audiostream_t *stream);

src/duskdolphin/audio/audiostreamplatform.h

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+/**
+ * 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