Blink the Pico onboard LED at 1 Hz when the connected DualSense
reports PowerPercent <= 1 (i.e. <= 10%) and PowerState == Discharging.
Source data is byte 52 of the BT 0x31 input report, already copied
into interrupt_in_data; no new BT parsing required.
The new module owns the LED only while it is actively blinking; it
detects controller disconnection via stale-report timeout and steps
out, so bt.cpp's existing connect/disconnect LED handling stays in
charge in all other states. Honors disable_pico_led.
Gated by -DENABLE_BATT_LED=ON (default). With the option off, the
source file is not compiled and behavior is identical to upstream.
CI gains a compile-only check for the OFF flavor.
(cherry picked from commit 2f8ea73c9fb695e24e7cc3329db7cb925e82e1c9)
