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/usr/sbin/i2c_keepalive.sh

@@ -0,0 +1,30 @@
+#!/bin/sh
+# i2c_keepalive.sh — X6200 AXP PMU keepalive
+#
+# The X6200 uses an AXP PMU on I2C bus 0, address 0x72.
+# Without periodic I2C traffic the unit powers off after ~180s.
+# This script sends a combined write+read transaction every second.
+#
+# Requires: i2c-tools (i2ctransfer)
+
+BUS=0
+ADDR=0x72
+INTERVAL=1
+
+# Only allow one instance
+if pidof -x "$(basename "$0")" >/dev/null 2>&1; then
+    COUNT="$(pidof -x "$(basename "$0")" | wc -w)"
+    [ "$COUNT" -gt 1 ] && { echo "[keepalive] already running"; exit 0; }
+fi
+
+trap 'exit 0' INT TERM
+
+# One-time kick at startup
+i2ctransfer -y "$BUS" w4@"$ADDR" 0xFF 0xFE 0x02 0x80 >/dev/null 2>&1
+i2ctransfer -y "$BUS" w4@"$ADDR" 0xFF 0xFE 0x03 0x80 >/dev/null 2>&1
+
+# Main keepalive loop
+while :; do
+    i2ctransfer -y "$BUS" w2@"$ADDR" 0x20 0x00 r1@"$ADDR" >/dev/null 2>&1
+    sleep "$INTERVAL"
+done