[EInkDisplay] Add basic implementation for displayWindow (#9)

Adds a basic implementation of `displayWindow` which displays a subset
of the current frame buffer to the screen. It is significantly less
performant than a full screen reload, so it shouldn't be used unless the
underlying content is hard to get/rerender.

I've also spotted a little bit of ghosting, but it's not fully clear as
to why that is. My only thought it the LUT and grayscale mode isn't
fully disengaged or the RED RAM buffer isn't correctly setup (but afaict
it should be populated correctly).

Going to merge this to continue testing this feature out in the wild,
but consider it experimental.

libs/display/EInkDisplay/include/EInkDisplay.h

@@ -43,6 +43,8 @@ class EInkDisplay {
 #endif
 
   void displayBuffer(RefreshMode mode = FAST_REFRESH);
+  // EXPERIMENTAL: Windowed update - display only a rectangular region
+  void displayWindow(uint16_t x, uint16_t y, uint16_t w, uint16_t h);
   void displayGrayBuffer(bool turnOffScreen = false);
 
   void refreshDisplay(RefreshMode mode = FAST_REFRESH, bool turnOffScreen = false);

libs/display/EInkDisplay/src/EInkDisplay.cpp

@@ -452,6 +452,82 @@ void EInkDisplay::displayBuffer(RefreshMode mode) {
 #endif
 }
 
+// EXPERIMENTAL: Windowed update support
+// Displays only a rectangular region of the frame buffer, preserving the rest of the screen.
+// Requirements: x and w must be byte-aligned (multiples of 8 pixels)
+void EInkDisplay::displayWindow(uint16_t x, uint16_t y, uint16_t w, uint16_t h) {
+  Serial.printf("[%lu]   Displaying window at (%d,%d) size (%dx%d)\n", millis(), x, y, w, h);
+
+  // Validate bounds
+  if (x + w > DISPLAY_WIDTH || y + h > DISPLAY_HEIGHT) {
+    Serial.printf("[%lu]   ERROR: Window bounds exceed display dimensions!\n", millis());
+    return;
+  }
+
+  // Validate byte alignment
+  if (x % 8 != 0 || w % 8 != 0) {
+    Serial.printf("[%lu]   ERROR: Window x and width must be byte-aligned (multiples of 8)!\n", millis());
+    return;
+  }
+
+  if (!frameBuffer) {
+    Serial.printf("[%lu]   ERROR: Frame buffer not allocated!\n", millis());
+    return;
+  }
+
+  // displayWindow is not supported while the rest of the screen has grayscale content, revert it
+  if (inGrayscaleMode) {
+    inGrayscaleMode = false;
+    grayscaleRevert();
+  }
+
+  // Calculate window buffer size
+  const uint16_t windowWidthBytes = w / 8;
+  const uint32_t windowBufferSize = windowWidthBytes * h;
+
+  Serial.printf("[%lu]   Window buffer size: %lu bytes (%d x %d pixels)\n", millis(), windowBufferSize, w, h);
+
+  // Allocate temporary buffer on stack
+  std::vector<uint8_t> windowBuffer(windowBufferSize);
+
+  // Extract window region from frame buffer
+  for (uint16_t row = 0; row < h; row++) {
+    const uint16_t srcY = y + row;
+    const uint16_t srcOffset = srcY * DISPLAY_WIDTH_BYTES + (x / 8);
+    const uint16_t dstOffset = row * windowWidthBytes;
+    memcpy(&windowBuffer[dstOffset], &frameBuffer[srcOffset], windowWidthBytes);
+  }
+
+  // Configure RAM area for window
+  setRamArea(x, y, w, h);
+
+  // Write to BW RAM (current frame)
+  writeRamBuffer(CMD_WRITE_RAM_BW, windowBuffer.data(), windowBufferSize);
+
+#ifndef EINK_DISPLAY_SINGLE_BUFFER_MODE
+  // Dual buffer: Extract window from frameBufferActive (previous frame)
+  std::vector<uint8_t> previousWindowBuffer(windowBufferSize);
+  for (uint16_t row = 0; row < h; row++) {
+    const uint16_t srcY = y + row;
+    const uint16_t srcOffset = srcY * DISPLAY_WIDTH_BYTES + (x / 8);
+    const uint16_t dstOffset = row * windowWidthBytes;
+    memcpy(&previousWindowBuffer[dstOffset], &frameBufferActive[srcOffset], windowWidthBytes);
+  }
+  writeRamBuffer(CMD_WRITE_RAM_RED, previousWindowBuffer.data(), windowBufferSize);
+#endif
+
+  // Perform fast refresh
+  refreshDisplay(FAST_REFRESH);
+
+#ifdef EINK_DISPLAY_SINGLE_BUFFER_MODE
+  // Post-refresh: Sync RED RAM with current window (for next fast refresh)
+  setRamArea(x, y, w, h);
+  writeRamBuffer(CMD_WRITE_RAM_RED, windowBuffer.data(), windowBufferSize);
+#endif
+
+  Serial.printf("[%lu]   Window display complete\n", millis());
+}
+
 void EInkDisplay::displayGrayBuffer(const bool turnOffScreen) {
   drawGrayscale = false;
   inGrayscaleMode = true;