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/hal/HalDisplay.cpp

@@ -32,6 +32,13 @@ void HalDisplay::displayBuffer(HalDisplay::RefreshMode mode, bool turnOffScreen)
   einkDisplay.displayBuffer(convertRefreshMode(mode), turnOffScreen);
 }
 
+// EXPERIMENTAL: Display only a rectangular region
+void HalDisplay::displayWindow(uint16_t x, uint16_t y, uint16_t w, uint16_t h,
+                               HalDisplay::RefreshMode mode, bool turnOffScreen) {
+  (void)mode;  // EInkDisplay::displayWindow does not take mode yet
+  einkDisplay.displayWindow(x, y, w, h, turnOffScreen);
+}
+
 void HalDisplay::refreshDisplay(HalDisplay::RefreshMode mode, bool turnOffScreen) {
   einkDisplay.refreshDisplay(convertRefreshMode(mode), turnOffScreen);
 }

lib/hal/HalDisplay.h

@@ -34,6 +34,10 @@ class HalDisplay {
   void displayBuffer(RefreshMode mode = RefreshMode::FAST_REFRESH, bool turnOffScreen = false);
   void refreshDisplay(RefreshMode mode = RefreshMode::FAST_REFRESH, bool turnOffScreen = false);
 
+  // EXPERIMENTAL: Display only a rectangular region
+  void displayWindow(uint16_t x, uint16_t y, uint16_t w, uint16_t h,
+                     RefreshMode mode = RefreshMode::FAST_REFRESH, bool turnOffScreen = false);
+
   // Power management
   void deepSleep();