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/audiopsp.h

@@ -0,0 +1,30 @@
+/**
+ * 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 PSP-specific audio subsystem state.
+ *
+ * @return Error state if any.
+ */
+errorret_t audioPSPInit();
+
+/**
+ * Updates the PSP-specific audio subsystem state. Called once per frame.
+ *
+ * @return Error state if any.
+ */
+errorret_t audioPSPUpdate();
+
+/**
+ * Disposes the PSP-specific audio subsystem state.
+ *
+ * @return Error state if any.
+ */
+errorret_t audioPSPDispose();