perf: Eliminate per-pixel overheads in image rendering (#1293)

## Summary

Replace per-pixel getRenderMode() + rotateCoordinates() + bounds checks
with a DirectPixelWriter struct that pre-computes orientation and render
mode state once per row. Use bitwise ops instead of division/modulo for
cache pixel packing. Skip PNG cache allocation when buffer exceeds 48KB
(framebuffer size) since PNG decode is fast enough that caching provides
minimal benefit, and the large buffer competes with the 44KB PNG decoder
for heap.

## Additional Context
Measured improvements on ESP32-C3 @ 160MHz:
- JPEG decode: 5-7% faster (1:1 scale)
- PNG decode: 15-20% faster (1:1 scale)
- Cache renders: 3-6% faster across both formats
- Eliminates "Failed to allocate cache buffer" errors for large PNGs

---

### AI Usage

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please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**<  PARTIALLY >**_

lib/Epub/Epub/blocks/ImageBlock.cpp

@@ -4,7 +4,7 @@
 #include <Logging.h>
 #include <Serialization.h>
 
-#include "../converters/DitherUtils.h"
+#include "../converters/DirectPixelWriter.h"
 #include "../converters/ImageDecoderFactory.h"
 
 // Cache file format:
@@ -66,6 +66,9 @@ bool renderFromCache(GfxRenderer& renderer, const std::string& cachePath, int x,
     return false;
   }
 
+  DirectPixelWriter pw;
+  pw.init(renderer);
+
   for (int row = 0; row < cachedHeight; row++) {
     if (cacheFile.read(rowBuffer, bytesPerRow) != bytesPerRow) {
       LOG_ERR("IMG", "Cache read error at row %d", row);
@@ -74,13 +77,14 @@ bool renderFromCache(GfxRenderer& renderer, const std::string& cachePath, int x,
       return false;
     }
 
-    int destY = y + row;
+    const int destY = y + row;
+    pw.beginRow(destY);
     for (int col = 0; col < cachedWidth; col++) {
-      int byteIdx = col / 4;
-      int bitShift = 6 - (col % 4) * 2;  // MSB first within byte
+      const int byteIdx = col >> 2;            // col / 4
+      const int bitShift = 6 - (col & 3) * 2;  // MSB first within byte
       uint8_t pixelValue = (rowBuffer[byteIdx] >> bitShift) & 0x03;
 
-      drawPixelWithRenderMode(renderer, x + col, destY, pixelValue);
+      pw.writePixel(x + col, pixelValue);
     }
   }