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/Epub/Epub/blocks/ImageBlock.cpp

@@ -0,0 +1,175 @@
+#include "ImageBlock.h"
+
+#include <FsHelpers.h>
+#include <GfxRenderer.h>
+#include <HardwareSerial.h>
+#include <SDCardManager.h>
+#include <Serialization.h>
+
+#include "../converters/DitherUtils.h"
+#include "../converters/ImageDecoderFactory.h"
+
+// Cache file format:
+// - uint16_t width
+// - uint16_t height
+// - uint8_t pixels[...] - 2 bits per pixel, packed (4 pixels per byte), row-major order
+
+ImageBlock::ImageBlock(const std::string& imagePath, int16_t width, int16_t height)
+    : imagePath(imagePath), width(width), height(height) {}
+
+bool ImageBlock::imageExists() const { return Storage.exists(imagePath.c_str()); }
+
+namespace {
+
+std::string getCachePath(const std::string& imagePath) {
+  // Replace extension with .pxc (pixel cache)
+  size_t dotPos = imagePath.rfind('.');
+  if (dotPos != std::string::npos) {
+    return imagePath.substr(0, dotPos) + ".pxc";
+  }
+  return imagePath + ".pxc";
+}
+
+bool renderFromCache(GfxRenderer& renderer, const std::string& cachePath, int x, int y, int expectedWidth,
+                     int expectedHeight) {
+  FsFile cacheFile;
+  if (!Storage.openFileForRead("IMG", cachePath, cacheFile)) {
+    return false;
+  }
+
+  uint16_t cachedWidth, cachedHeight;
+  if (cacheFile.read(&cachedWidth, 2) != 2 || cacheFile.read(&cachedHeight, 2) != 2) {
+    cacheFile.close();
+    return false;
+  }
+
+  // Verify dimensions are close (allow 1 pixel tolerance for rounding differences)
+  int widthDiff = abs(cachedWidth - expectedWidth);
+  int heightDiff = abs(cachedHeight - expectedHeight);
+  if (widthDiff > 1 || heightDiff > 1) {
+    Serial.printf("[%lu] [IMG] Cache dimension mismatch: %dx%d vs %dx%d\n", millis(), cachedWidth, cachedHeight,
+                  expectedWidth, expectedHeight);
+    cacheFile.close();
+    return false;
+  }
+
+  // Use cached dimensions for rendering (they're the actual decoded size)
+  expectedWidth = cachedWidth;
+  expectedHeight = cachedHeight;
+
+  Serial.printf("[%lu] [IMG] Loading from cache: %s (%dx%d)\n", millis(), cachePath.c_str(), cachedWidth, cachedHeight);
+
+  // Read and render row by row to minimize memory usage
+  const int bytesPerRow = (cachedWidth + 3) / 4;  // 2 bits per pixel, 4 pixels per byte
+  uint8_t* rowBuffer = (uint8_t*)malloc(bytesPerRow);
+  if (!rowBuffer) {
+    Serial.printf("[%lu] [IMG] Failed to allocate row buffer\n", millis());
+    cacheFile.close();
+    return false;
+  }
+
+  for (int row = 0; row < cachedHeight; row++) {
+    if (cacheFile.read(rowBuffer, bytesPerRow) != bytesPerRow) {
+      Serial.printf("[%lu] [IMG] Cache read error at row %d\n", millis(), row);
+      free(rowBuffer);
+      cacheFile.close();
+      return false;
+    }
+
+    int destY = y + row;
+    for (int col = 0; col < cachedWidth; col++) {
+      int byteIdx = col / 4;
+      int bitShift = 6 - (col % 4) * 2;  // MSB first within byte
+      uint8_t pixelValue = (rowBuffer[byteIdx] >> bitShift) & 0x03;
+
+      drawPixelWithRenderMode(renderer, x + col, destY, pixelValue);
+    }
+  }
+
+  free(rowBuffer);
+  cacheFile.close();
+  Serial.printf("[%lu] [IMG] Cache render complete\n", millis());
+  return true;
+}
+
+}  // namespace
+
+void ImageBlock::render(GfxRenderer& renderer, const int x, const int y) {
+  Serial.printf("[%lu] [IMG] Rendering image at %d,%d: %s (%dx%d)\n", millis(), x, y, imagePath.c_str(), width, height);
+
+  const int screenWidth = renderer.getScreenWidth();
+  const int screenHeight = renderer.getScreenHeight();
+
+  // Bounds check render position using logical screen dimensions
+  if (x < 0 || y < 0 || x + width > screenWidth || y + height > screenHeight) {
+    Serial.printf("[%lu] [IMG] Invalid render position: (%d,%d) size (%dx%d) screen (%dx%d)\n", millis(), x, y, width,
+                  height, screenWidth, screenHeight);
+    return;
+  }
+
+  // Try to render from cache first
+  std::string cachePath = getCachePath(imagePath);
+  if (renderFromCache(renderer, cachePath, x, y, width, height)) {
+    return;  // Successfully rendered from cache
+  }
+
+  // No cache - need to decode the image
+  // Check if image file exists
+  FsFile file;
+  if (!Storage.openFileForRead("IMG", imagePath, file)) {
+    Serial.printf("[%lu] [IMG] Image file not found: %s\n", millis(), imagePath.c_str());
+    return;
+  }
+  size_t fileSize = file.size();
+  file.close();
+
+  if (fileSize == 0) {
+    Serial.printf("[%lu] [IMG] Image file is empty: %s\n", millis(), imagePath.c_str());
+    return;
+  }
+
+  Serial.printf("[%lu] [IMG] Decoding and caching: %s\n", millis(), imagePath.c_str());
+
+  RenderConfig config;
+  config.x = x;
+  config.y = y;
+  config.maxWidth = width;
+  config.maxHeight = height;
+  config.useGrayscale = true;
+  config.useDithering = true;
+  config.performanceMode = false;
+  config.useExactDimensions = true;  // Use pre-calculated dimensions to avoid rounding mismatches
+  config.cachePath = cachePath;      // Enable caching during decode
+
+  ImageToFramebufferDecoder* decoder = ImageDecoderFactory::getDecoder(imagePath);
+  if (!decoder) {
+    Serial.printf("[%lu] [IMG] No decoder found for image: %s\n", millis(), imagePath.c_str());
+    return;
+  }
+
+  Serial.printf("[%lu] [IMG] Using %s decoder\n", millis(), decoder->getFormatName());
+
+  bool success = decoder->decodeToFramebuffer(imagePath, renderer, config);
+  if (!success) {
+    Serial.printf("[%lu] [IMG] Failed to decode image: %s\n", millis(), imagePath.c_str());
+    return;
+  }
+
+  Serial.printf("[%lu] [IMG] Decode successful\n", millis());
+}
+
+bool ImageBlock::serialize(FsFile& file) {
+  serialization::writeString(file, imagePath);
+  serialization::writePod(file, width);
+  serialization::writePod(file, height);
+  return true;
+}
+
+std::unique_ptr<ImageBlock> ImageBlock::deserialize(FsFile& file) {
+  std::string path;
+  serialization::readString(file, path);
+  int16_t w, h;
+  serialization::readPod(file, w);
+  serialization::readPod(file, h);
+  return std::unique_ptr<ImageBlock>(new ImageBlock(path, w, h));
+}