🖼️ Grayscale Rendering Boost — Release Notes

What changed: a big improvement in how photos and images are shown on your e-reader's grayscale display — especially screensaver / sleep screen images and book covers on the X4/X3 panels.

🌙 Screensaver Fixes — Release Notes

What changed: two small but noticeable "quality of life" fixes for the
screensaver — waking it up behaves the way it should, and the shuffle doesn't
keep showing the same image first.

🌙 No more refresh when waking the screensaver

Previously, if your power button was set to "refresh screen", waking the screensaver with a single press could also run a manual screen refresh — a flicker you didn't ask for.

📌 Summary

• ✅ Real contrast, not washed-out — images now look closer to what you see on a computer, instead of pale and flat.
• ✅ Safer, crash-free rendering — rewritten the math with explicit 16-bit variables and guarded buffers (no more negative-index writes that can corrupt memory on ESP32).
• ✅ One shared engine — the BMP reader, JPEG/PNG covers and screensaver/sleep all use the same tuned settings, so results are consistent everywhere an image appears.
• ✅ Faster startup — the gamma correction table is prepared once at boot.

🔍 Under the hood (brief)

The e-ink display uses only 4 shades of gray. Old code pushed pixels through with flat quantization, which made dark images lose their shadows and highlights look washed out.

Now the pipeline is:

1. 🎛️ Gamma correction — lifts midtones so the 4 gray levels separate cleanly (tuned to gamma 1.5).
2. ✂️ Empirical thresholds 50 / 120 / 200 — chosen specifically for the X4 panel, so each shade lands where the eye expects it.
3. 🗺️ Error diffusion dithering — Atkinson (and the serial Floyd-Steinberg fallback) spreads the "error" of each quantized pixel to its neighbors, producing smooth gradients instead of banding.
4. 🛡️ Overflow-safe math — everything runs in int16_t with clamped values and offset, bounds-checked buffers. No floats in the hot loop, so it's both safe and fast on the ESP32-C3.
5. 🧵 Clean row edges — horizontal error is dropped at the end of each row, preventing vertical streak artifacts.

🛡️ Stability & safety improvements

• All per-pixel math in int16_t — far within range for X4-sized rows.
• Error buffers allocated once per image and reused across rows — no memory churn, no per-line allocations.
• No negative index access — full audit confirms the buffers never go before index 0 (would otherwise corrupt memory on ESP32).
• Lazy-init guard on the gamma table — even if a startup hook is skipped, images can never render as a blank black screen.

📋 Files changed

• 🆕 lib/GfxRenderer/DitheringConfig.h — new shared configuration
• lib/GfxRenderer/BitmapHelpers.{h,cpp} — ditherers + quantization
• lib/GfxRenderer/Bitmap.cpp
• lib/JpegToBmpConverter/JpegToBmpConverter.cpp
• lib/PngToBmpConverter/PngToBmpConverter.cpp
• src/main.cpp — gamma LUT init at boot