Add async display refresh support to HalDisplay

Introduces non-blocking refresh capability with displayBufferAsync() that starts panel waveform and returns immediately, plus waitRefreshComplete() to block until refresh finishes. Panels without async support fall back to blocking refresh. Requires framebuffer to remain untouched during refresh and differential baseline rebuild before next update.

lib/hal/HalDisplay.cpp

@@ -65,6 +65,18 @@ void HalDisplay::displayBuffer(HalDisplay::RefreshMode mode, bool turnOffScreen)
   einkDisplay.displayBuffer(convertRefreshMode(mode), turnOffScreen);
 }
 
+void HalDisplay::displayBufferAsync(HalDisplay::RefreshMode mode) {
+  if (gpio.deviceIsX3() && mode == RefreshMode::HALF_REFRESH) {
+    einkDisplay.requestResync(1);
+  }
+
+  einkDisplay.displayBufferAsyncNoShadow(convertRefreshMode(mode));
+}
+
+void HalDisplay::waitRefreshComplete() { einkDisplay.waitRefreshComplete(); }
+
+bool HalDisplay::supportsAsyncRefresh() const { return einkDisplay.supportsAsyncRefresh(); }
+
 void HalDisplay::refreshDisplay(HalDisplay::RefreshMode mode, bool turnOffScreen) {
   if (gpio.deviceIsX3() && mode == RefreshMode::HALF_REFRESH) {
     einkDisplay.requestResync(1);

lib/hal/HalDisplay.h

@@ -39,6 +39,17 @@ class HalDisplay {
                             bool fromProgmem = false) const;
 
   void displayBuffer(RefreshMode mode = RefreshMode::FAST_REFRESH, bool turnOffScreen = false);
+  // Non-blocking refresh (shadow-free): starts the panel waveform and returns
+  // while the panel refreshes on its own. The framebuffer must stay untouched
+  // until waitRefreshComplete(), and the caller must rebuild the differential
+  // baseline before the next differential update (the tiled grayscale cleanup
+  // does). Panels without async support fall back to a blocking refresh.
+  void displayBufferAsync(RefreshMode mode = RefreshMode::FAST_REFRESH);
+  // Block until a pending async refresh completes (no-op when none is).
+  void waitRefreshComplete();
+  // True when displayBufferAsync() genuinely overlaps (panel driver has real
+  // async support); false where it falls back to a blocking refresh.
+  bool supportsAsyncRefresh() const;
   void refreshDisplay(RefreshMode mode = RefreshMode::FAST_REFRESH, bool turnOffScreen = false);
 
   // Power management