feat: Add EPUB embedded image support (JPEG/PNG)

Cherry-pick merge from pablohc/crosspoint-reader@2d8cbcf, based on
upstream PR #556 by martinbrook with pablohc's refresh optimization.

- Add JPEG decoder (picojpeg) and PNG decoder (PNGdec) with 4-level
  grayscale Bayer dithering for e-ink display
- Add pixel caching system (.pxc files) for fast image re-rendering
- Integrate image extraction from EPUB HTML parser (<img> tag support)
- Add ImageBlock/PageImage types with serialization support
- Add image-aware refresh optimization (double FAST_REFRESH technique)
- Add experimental displayWindow() partial refresh support
- Bump section cache version 12->13 to invalidate stale caches
- Resolve TAG_PageImage=3 to avoid conflict with mod's TAG_PageTableRow=2

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/GfxRenderer/GfxRenderer.h

@@ -70,7 +70,8 @@ class GfxRenderer {
   int getScreenHeight() const;
   void displayBuffer(HalDisplay::RefreshMode refreshMode = HalDisplay::FAST_REFRESH) const;
   // EXPERIMENTAL: Windowed update - display only a rectangular region
-  // void displayWindow(int x, int y, int width, int height) const;
+  void displayWindow(int x, int y, int width, int height,
+                     HalDisplay::RefreshMode mode = HalDisplay::FAST_REFRESH) const;
   void invertScreen() const;
   void clearScreen(uint8_t color = 0xFF) const;
   void getOrientedViewableTRBL(int* outTop, int* outRight, int* outBottom, int* outLeft) const;
@@ -120,6 +121,7 @@ class GfxRenderer {
 
   // Grayscale functions
   void setRenderMode(const RenderMode mode) { this->renderMode = mode; }
+  RenderMode getRenderMode() const { return renderMode; }
   void copyGrayscaleLsbBuffers() const;
   void copyGrayscaleMsbBuffers() const;
   void displayGrayBuffer() const;