feat: tiled grayscale rendering to drop the storeBwBuffer peak (#2106)

Tiled grayscale rendering to drop the storeBwBuffer peak largest
contiguous free block (the value that actually drives OOM on the C3)
from ~114 KB to ~82-90 KB.

This renders each grayscale plane band-by-band into a small (~8 KB)
scratch and
streams each band straight to controller RAM (community-sdk
writeGrayscalePlaneStrip), leaving the BW framebuffer intact. No save,
no
restore; controller RAM is re-synced for the next differential turn
directly
from the live framebuffer.

Three writers honor the active band target so per-band re-rendering
stays cheap
and correct:

- drawPixel (text) redirects writes to the band scratch and clips to it.
- renderCharImpl skips glyphs whose physical y-extent is outside the
band before
the bitmap decode (glyphIntersectsStrip), so the per-band re-render
doesn't
  pay N x glyph decode.
- DirectPixelWriter (images) writes the band scratch via getWriteTarget
instead
  of the framebuffer. Without this, image pixels wrote the live BW frame
directly and cleanup re-synced that corruption, leaving thin outlines
after
  navigating away from an image.

Controller specifics live in the SDK (X4 setRamArea windowing, X3 PTL);
the
reader checks supportsStripGrayscale() and is otherwise
controller-agnostic.

Measured on hardware (X4 and X3, text and images, visually correct):

- Grayscale scratch ~8 KB vs ~50 KB save; largest contiguous free block
held at
  full size during grayscale instead of dropping ~25-32 KB.
- X4 text page about +25 ms/page; X3 page time is dominated by its
intrinsic
  grayscale refresh, not tiling.

Depends on community-sdk #13 (the writeGrayscalePlaneStrip API). The
submodule
bump here points at that branch, so until #13 merges the submodule won't
resolve
from upstream and CI will fail there; keeping this a draft until then.
Will
rebase onto master and re-point the submodule to the merged SDK commit
once #13
lands.

Did you use AI tools to help write this code? partial

lib/hal/HalDisplay.cpp

@@ -89,6 +89,13 @@ void HalDisplay::cleanupGrayscaleBuffers(const uint8_t* bwBuffer) { einkDisplay.
 
 void HalDisplay::displayGrayBuffer(bool turnOffScreen) { einkDisplay.displayGrayBuffer(turnOffScreen); }
 
+void HalDisplay::writeGrayscalePlaneStrip(bool lsbPlane, const uint8_t* rows, uint16_t yStart, uint16_t numRows) {
+  einkDisplay.writeGrayscalePlaneStrip(lsbPlane ? EInkDisplay::GRAY_PLANE_LSB : EInkDisplay::GRAY_PLANE_MSB, rows,
+                                       yStart, numRows);
+}
+
+bool HalDisplay::supportsStripGrayscale() const { return einkDisplay.supportsStripGrayscale(); }
+
 uint16_t HalDisplay::getDisplayWidth() const { return einkDisplay.getDisplayWidth(); }
 
 uint16_t HalDisplay::getDisplayHeight() const { return einkDisplay.getDisplayHeight(); }