feat: battery indicator and rumble test button

Plumb controller battery type/level from the GIP status packets
through the C API into the card header (SF Symbol battery icon).
Add a per-controller Rumble button wired to a new
xone_controller_rumble() entry point; it validates its arguments
and returns -ENOSYS until host to controller TX is implemented.

Co-Authored-By: qwen3.8-27b@q3_k_xl: battery + rumble button UI

include/app/xone_api.h

@@ -74,7 +74,8 @@ int xone_controller_mac(xone_dongle const *d, int index, char *buf, int len);
 
 // Snapshot of the latest standard Xbox controller input report. Stick values
 // are signed 16-bit values; trigger values are unsigned 10-bit values. The
-// buttons field uses the standard GIP gamepad bit assignments.
+// buttons field uses the standard GIP gamepad bit assignments. Battery fields
+// come from periodic GIP status packets (battery_type is 0 when unknown).
 struct xone_controller_state {
     uint16_t buttons;
     bool guide_down;
@@ -87,6 +88,8 @@ struct xone_controller_state {
     int16_t stick_right_x;
     int16_t stick_right_y;
     uint32_t sequence;
+    uint8_t battery_type;
+    uint8_t battery_level;
 };
 
 // Copy the latest input state for the controller at index. Returns 0 on
@@ -97,6 +100,12 @@ int xone_controller_get_state(xone_dongle const *d, int index,
 // Enter or leave pairing mode (beacon pairing flag + LED blink).
 void xone_set_pairing(xone_dongle *d, bool enable);
 
+// Trigger a rumble test on the controller at index. Returns 0 on success,
+// -1 for an invalid session or index. Not implemented yet: host to
+// controller TX (GIP_CMD_RUMBLE payload) is pending, so this currently
+// returns -ENOSYS.
+int xone_controller_rumble(xone_dongle *d, int index);
+
 #ifdef __cplusplus
 }
 #endif