fix(radio): keep the radio serviced while an e-ink refresh blocks the loop

GxEPD2 polls the panel's BUSY pin for the whole refresh (up to ~1-2s),
stalling the main loop. A TX finishing in that window left the SX126x in
standby -- deaf -- until the refresh ended; packets received meanwhile could
be lost (only the latest is readable, and a later CRC failure rejects a good
one still pending). Hook GxEPD2's busy callback to finish TX / drain RX into
a small staging queue and re-arm right away, without touching dispatch/UI.
Wired only where the display driver supports it (GxEPD2: the two e-ink Solo
boards); OLED and sim builds are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/helpers/ui/GxEPDDisplay.cpp

@@ -84,6 +84,10 @@ bool GxEPDDisplay::begin() {
   SPI1.begin();
 #endif
   display.init(115200, true, 2, false);
+  // Runs ~every 1ms while _waitWhileBusy() polls the panel's BUSY pin, i.e.
+  // for as long as a refresh blocks the main loop. See busyCallbackTrampoline
+  // and callBusyPump()/setBusyPumpFn() (DisplayDriver.h).
+  display.epd2.setBusyCallback(busyCallbackTrampoline, this);
   display.setRotation(DISPLAY_ROTATION);
   setTextSize(1);
   display.setPartialWindow(0, 0, display.width(), display.height());