Split audio into stream/ and mixer/ subdirs, fix resulting build breaks

Reorganizes src/dusk/audio/: individual stream implementations
(audiostream, audiostreampcm, audiostreammp3, audiostreammp3decodersw)
move into audio/stream/, and a new audio/mixer/ holds a channel-based
playback queue (audiomixer.c/.h) for future use - not yet wired into
the engine.

Fixes needed to keep the tree buildable after the move:
 - A stray duplicate of audiostreammp3decodersw.c/.h was left at the old
   flat path; removed in favor of the canonical copy in stream/.
 - Updated every #include "audio/audiostream*.h" and the two platform
   CMakeLists.txt (dusklinux, duskdolphin) that still pointed at the old
   flat location.
 - audiomixer.c/.h didn't compile: audiomixerqueue_t was referenced but
   never defined (audiomixerchanneldata_t has the matching fields), a
   trailing comma in audioMixerPlayLooped's parameter list is illegal in
   C, and `file` was declared as an array of pointers instead of a char
   buffer.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

src/duskpsp/audio/audiostreammp3decoder.c

@@ -6,7 +6,7 @@
  */
 
 #include "audiostreammp3decoder.h"
-#include "audio/audiostream.h"
+#include "audio/stream/audiostream.h"
 #include "assert/assert.h"
 #include "util/memory.h"
 #include <pspmp3.h>

src/duskpsp/audio/audiostreampsp.c

@@ -6,7 +6,7 @@
  */
 
 #include "audiostreampsp.h"
-#include "audio/audiostream.h"
+#include "audio/stream/audiostream.h"
 #include "assert/assert.h"
 #include "util/memory.h"
 #include "util/math.h"