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/dusklinux/audio/CMakeLists.txt

@@ -0,0 +1,11 @@
+# Copyright (c) 2026 Dominic Masters
+#
+# This software is released under the MIT License.
+# https://opensource.org/licenses/MIT
+
+# Sources
+target_sources(${DUSK_LIBRARY_TARGET_NAME}
+  PUBLIC
+    audiolinux.c
+    audiostreamlinux.c
+)