[client] usb audio: add audio provider

Adapt the emulated UAC2 playback stream to LG_AudioOps.

Preserve borrowed packet delivery, publish reconstructed timing, and
synchronize provider availability and attachment transitions.

client/src/audio_usb.h

@@ -0,0 +1,37 @@
+/**
+ * Looking Glass
+ * Copyright © 2017-2026 The Looking Glass Authors
+ * https://looking-glass.io
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc., 59
+ * Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ */
+
+#ifndef _H_LG_CLIENT_AUDIO_USB_
+#define _H_LG_CLIENT_AUDIO_USB_
+
+#include "interface/audio.h"
+#include "usbredir.h"
+
+typedef struct LGA_USBState LGA_USBState;
+
+LGA_USBState * lgaUsb_create(void);
+/* Detach this provider and stop PureSpice before destroying its state. */
+void lgaUsb_destroy(LGA_USBState * state);
+
+LG_USBRedir * lgaUsb_redir(LGA_USBState * state);
+
+extern const LG_AudioOps LGA_USB;
+
+#endif