feat: audio stack working — pulseaudio + x6200_control + voice_rec

- Add pulseaudio in system mode
- Add alsactl restore with correct mixer state from Aether
- Add x6200-control package (I2C hardware control library)
- Audio chain: pulseaudio -> x6200_control_init -> voice_rec -> speaker

br2_external/board/x6200/rootfs-overlay/etc/init.d/S96keepalive

@@ -0,0 +1,23 @@
+#!/bin/sh
+# S96keepalive — X6200 PMU keepalive
+#
+# The X6200 powers itself off after ~180s unless the AXP PMU receives
+# periodic I2C traffic. This script starts the keepalive daemon at boot.
+
+case "$1" in
+    start)
+        echo "Starting keepalive"
+        /usr/sbin/i2c_keepalive.sh >/dev/null 2>&1 &
+        ;;
+    stop)
+        echo "Stopping keepalive"
+        killall i2c_keepalive.sh 2>/dev/null || true
+        ;;
+    restart)
+        $0 stop; sleep 1; $0 start
+        ;;
+    *)
+        echo "Usage: $0 {start|stop|restart}" >&2
+        exit 1
+        ;;
+esac