Battery voltage fluctuates near the full threshold, causing the charge status light to flicker between red (charging) and green (full).

Changes

• LIION_FULL_HYSTERESIS_MV macro — 100 mV (50 mV/cell) hysteresis band around the full threshold
• pbsys_battery_is_full() hysteresis logic — uses a static bool was_full to implement a Schmitt-trigger-style check: transitions to full at LIION_FULL_MV (8190 mV), stays full until voltage drops below LIION_FULL_MV - LIION_FULL_HYSTERESIS_MV (8090 mV)

bool pbsys_battery_is_full(void) {
    static bool was_full;
    int32_t voltage = pbio_battery_get_average_voltage();
    if (was_full) {
        was_full = voltage >= LIION_FULL_MV - LIION_FULL_HYSTERESIS_MV;
    } else {
        was_full = voltage >= LIION_FULL_MV;
    }
    return was_full;
}

The static variable is zero-initialized by the C standard, so on first call the strict >= LIION_FULL_MV threshold applies — correctly detecting a battery that is already full at boot.