Add XTC/XTCH ebook format support (#135)

## Summary

* **What is the goal of this PR?**

Add support for XTC (XTeink X4 native) ebook format, which contains
pre-rendered 480x800 1-bit bitmap pages optimized for e-ink displays.

* **What changes are included?**

- New `lib/Xtc/` library with XtcParser for reading XTC files
- XtcReaderActivity for displaying XTC pages on e-ink display
- XTC file detection in FileSelectionActivity
- Cover BMP generation from first XTC page
- Correct XTG page header structure (22 bytes) and bit polarity handling

## Additional Context

- XTC files contain pre-rendered bitmap pages with embedded status bar
(page numbers, progress %)
- XTG page header: 22 bytes (magic + dimensions + reserved fields +
bitmap size)
- Bit polarity: 0 = black, 1 = white
- No runtime text rendering needed - pages display directly on e-ink
- Faster page display compared to EPUB since no parsing/rendering
required
- Memory efficient: loads one page at a time (48KB per page)
- Tested with XTC files generated from https://x4converter.rho.sh/
- Verified correct page alignment and color rendering
- Please report any issues if you test with XTC files from other
sources.

---------

Co-authored-by: Dave Allie <dave@daveallie.com>

lib/Xtc/README

@@ -0,0 +1,40 @@
+# XTC/XTCH Library
+
+XTC ebook format support for CrossPoint Reader.
+
+## Supported Formats
+
+| Format | Extension | Description                                  |
+|--------|-----------|----------------------------------------------|
+| XTC    | `.xtc`    | Container with XTG pages (1-bit monochrome)  |
+| XTCH   | `.xtch`   | Container with XTH pages (2-bit grayscale)   |
+
+## Format Overview
+
+XTC/XTCH are container formats designed for ESP32 e-paper displays. They store pre-rendered bitmap pages optimized for the XTeink X4 e-reader (480x800 resolution).
+
+### Container Structure (XTC/XTCH)
+
+- 56-byte header with metadata offsets
+- Optional metadata (title, author, etc.)
+- Page index table (16 bytes per page)
+- Page data (XTG or XTH format)
+
+### Page Formats
+
+#### XTG (1-bit monochrome)
+
+- Row-major storage, 8 pixels per byte
+- MSB first (bit 7 = leftmost pixel)
+- 0 = Black, 1 = White
+
+#### XTH (2-bit grayscale)
+
+- Two bit planes stored sequentially
+- Column-major order (right to left)
+- 8 vertical pixels per byte
+- Grayscale: 0=White, 1=Dark Grey, 2=Light Grey, 3=Black
+
+## Reference
+
+Original format info: <https://gist.github.com/CrazyCoder/b125f26d6987c0620058249f59f1327d>