feat: low-battery LED indicator (ENABLE_BATT_LED, default ON)

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)

src/battery_led.cpp

@@ -0,0 +1,77 @@
+//
+// Low-battery LED indicator. See battery_led.h.
+//
+
+#include "battery_led.h"
+
+#include <cstdint>
+
+#include "config.h"
+#include "pico/cyw43_arch.h"
+#include "pico/time.h"
+
+extern uint8_t interrupt_in_data[63];
+
+namespace {
+
+constexpr uint64_t REPORT_STALE_US = 2'000'000;  // assume disconnected if no report for 2 s
+constexpr uint64_t BLINK_PERIOD_US =   500'000;  // 1 Hz, 50% duty
+constexpr uint8_t  THRESHOLD_LEVEL = 1;          // PowerPercent <= 1 (i.e. <= 10%)
+constexpr uint8_t  POWER_STATE_DISCHARGING = 0x0;
+
+uint64_t last_report_us = 0;
+uint64_t last_toggle_us = 0;
+bool     blinking       = false;
+bool     led_state      = false;
+
+}  // namespace
+
+void battery_led_init(void) {
+    last_report_us = 0;
+    last_toggle_us = 0;
+    blinking = false;
+    led_state = false;
+}
+
+void battery_led_note_report(void) {
+    last_report_us = time_us_64();
+}
+
+void battery_led_tick(void) {
+    if (get_config().disable_pico_led) {
+        blinking = false;
+        return;
+    }
+
+    const uint64_t now = time_us_64();
+    if (last_report_us == 0 || (now - last_report_us) >= REPORT_STALE_US) {
+        // No fresh data — bt.cpp owns the LED while disconnected.
+        blinking = false;
+        return;
+    }
+
+    const uint8_t b   = interrupt_in_data[52];
+    const uint8_t pct = b & 0x0F;
+    const uint8_t st  = (b >> 4) & 0x0F;
+    const bool low    = (st == POWER_STATE_DISCHARGING) && (pct <= THRESHOLD_LEVEL);
+
+    if (low) {
+        if (!blinking) {
+            blinking = true;
+            led_state = true;
+            last_toggle_us = now;
+            cyw43_arch_gpio_put(CYW43_WL_GPIO_LED_PIN, true);
+            return;
+        }
+        if ((now - last_toggle_us) >= BLINK_PERIOD_US) {
+            led_state = !led_state;
+            last_toggle_us = now;
+            cyw43_arch_gpio_put(CYW43_WL_GPIO_LED_PIN, led_state);
+        }
+    } else if (blinking) {
+        blinking = false;
+        // We were blinking and are still receiving fresh reports => still connected.
+        // Restore the LED to the bt.cpp "connected = solid on" state.
+        cyw43_arch_gpio_put(CYW43_WL_GPIO_LED_PIN, true);
+    }
+}