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

src/activities/reader/XtcReaderActivity.cpp

@@ -294,7 +294,19 @@ void XtcReaderActivity::renderPage() {
       }
     }
 
-    ReaderUtils::displayWithRefreshCycle(renderer, pagesUntilFullRefresh);
+    if (pagesUntilFullRefresh <= 1) {
+      // Periodic ghost cleanup: scrub via the normal path, then run the
+      // settle flavor of the grayscale base pass (DTM planes are equal after
+      // the display sync, so only the gentle reinforcement cells fire).
+      renderer.displayBuffer(HalDisplay::HALF_REFRESH);
+      renderer.preconditionGrayscale();
+      pagesUntilFullRefresh = SETTINGS.getRefreshFrequency();
+    } else {
+      // OEM grayscale pipeline base: differential "AA-pre-BW(mid)" update as
+      // the page turn on X3; plain FAST refresh on X4 (previous behavior).
+      renderer.displayGrayscaleBase(HalDisplay::FAST_REFRESH);
+      pagesUntilFullRefresh--;
+    }
 
     // Pass 2: LSB buffer - mark DARK gray only (XTH value 1)
     // In LUT: 0 bit = apply gray effect, 1 bit = untouched