feat: Add displayGrayscaleBase method for differential refresh

(#2334)

Introduces a new display method that prepares the framebuffer as a base
frame for grayscale overlays. On X3 panels, this uses the OEM
differential base waveform (AA-pre-BW) without forcing a resync. Other
panels fall back to normal display with configurable refresh mode.


Dependent upon matching SDK commit to work

lib/hal/HalDisplay.h

@@ -47,6 +47,19 @@ class HalDisplay {
   // Access to frame buffer
   uint8_t* getFrameBuffer() const;
 
+  // X3 grayscale preconditioning (OEM "AA-pre-BW(mid)" settle pass), windowed
+  // to the gray region in physical panel coordinates (no-arg = full frame).
+  // Call after the BW base frame is displayed and before the grayscale planes
+  // are written; no-op on X4. See EInkDisplay::preconditionGrayscale.
+  void preconditionGrayscale();
+  void preconditionGrayscale(uint16_t x, uint16_t y, uint16_t w, uint16_t h);
+
+  // Display the framebuffer as the base frame for a grayscale overlay that
+  // follows. X3 uses the OEM differential base waveform ("AA-pre-BW(mid)");
+  // other panels display normally with `fallback` mode (previous behavior).
+  // Deliberately does NOT force the X3 resync that displayBuffer(HALF) does.
+  void displayGrayscaleBase(RefreshMode fallback = HALF_REFRESH, bool turnOffScreen = false);
+
   void copyGrayscaleBuffers(const uint8_t* lsbBuffer, const uint8_t* msbBuffer);
   void copyGrayscaleLsbBuffers(const uint8_t* lsbBuffer);
   void copyGrayscaleMsbBuffers(const uint8_t* msbBuffer);