fix(batt-led): force LED off + disarm on disconnect

The low-battery blink would stay frozen mid-cycle (or briefly resume
during reconnect retries) after a controller died and lost connection,
because the stale-report check stopped toggling but never turned the
LED off, and the cached interrupt_in_data[52] still read low.

New `battery_led_on_disconnect()` clears blink state, forces the LED
off, and zeros last_report_us so the stale-check early-return blocks
any new blink until a fresh 0x31 report arrives on the next connection.
Called from bt.cpp's HCI_EVENT_DISCONNECTION_COMPLETE handler.

Stale-check in the tick also now forces LED off when it fires while a
blink was in progress (defense in depth for unclean disconnects).

Same bug exists in upstream awalol/DS5Dongle (their battery_led.cpp is
byte-identical to ours pre-fix) — Sura Academy reported it in their
Discord. Plan to send this back as a PR after we validate it locally.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/battery_led.h

@@ -16,3 +16,11 @@ void battery_led_tick(void);
 // has been copied into interrupt_in_data. Used to detect disconnection
 // via stale-report timeout.
 void battery_led_note_report(void);
+
+// Call from the BT disconnect handler. Cancels any in-progress blink,
+// forces the LED off, and arms the module so it ignores the cached
+// (now-stale) battery byte until a fresh report arrives on the next
+// connection. Without this, the LED can stay frozen in whichever state
+// it was at the moment of disconnect, or briefly resume blinking during
+// reconnect retries while interrupt_in_data[52] still reads low.
+void battery_led_on_disconnect(void);