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/Xtc/XtcParser.h

@@ -0,0 +1,96 @@
+/**
+ * XtcParser.h
+ *
+ * XTC file parsing and page data extraction
+ * XTC ebook support for CrossPoint Reader
+ */
+
+#pragma once
+
+#include <SD.h>
+
+#include <functional>
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "XtcTypes.h"
+
+namespace xtc {
+
+/**
+ * XTC File Parser
+ *
+ * Reads XTC files from SD card and extracts page data.
+ * Designed for ESP32-C3's limited RAM (~380KB) using streaming.
+ */
+class XtcParser {
+ public:
+  XtcParser();
+  ~XtcParser();
+
+  // File open/close
+  XtcError open(const char* filepath);
+  void close();
+  bool isOpen() const { return m_isOpen; }
+
+  // Header information access
+  const XtcHeader& getHeader() const { return m_header; }
+  uint16_t getPageCount() const { return m_header.pageCount; }
+  uint16_t getWidth() const { return m_defaultWidth; }
+  uint16_t getHeight() const { return m_defaultHeight; }
+  uint8_t getBitDepth() const { return m_bitDepth; }  // 1 = XTC/XTG, 2 = XTCH/XTH
+
+  // Page information
+  bool getPageInfo(uint32_t pageIndex, PageInfo& info) const;
+
+  /**
+   * Load page bitmap (raw 1-bit data, skipping XTG header)
+   *
+   * @param pageIndex Page index (0-based)
+   * @param buffer Output buffer (caller allocated)
+   * @param bufferSize Buffer size
+   * @return Number of bytes read on success, 0 on failure
+   */
+  size_t loadPage(uint32_t pageIndex, uint8_t* buffer, size_t bufferSize);
+
+  /**
+   * Streaming page load
+   * Memory-efficient method that reads page data in chunks.
+   *
+   * @param pageIndex Page index
+   * @param callback Callback function to receive data chunks
+   * @param chunkSize Chunk size (default: 1024 bytes)
+   * @return Error code
+   */
+  XtcError loadPageStreaming(uint32_t pageIndex,
+                             std::function<void(const uint8_t* data, size_t size, size_t offset)> callback,
+                             size_t chunkSize = 1024);
+
+  // Get title from metadata
+  std::string getTitle() const { return m_title; }
+
+  // Validation
+  static bool isValidXtcFile(const char* filepath);
+
+  // Error information
+  XtcError getLastError() const { return m_lastError; }
+
+ private:
+  File m_file;
+  bool m_isOpen;
+  XtcHeader m_header;
+  std::vector<PageInfo> m_pageTable;
+  std::string m_title;
+  uint16_t m_defaultWidth;
+  uint16_t m_defaultHeight;
+  uint8_t m_bitDepth;  // 1 = XTC/XTG (1-bit), 2 = XTCH/XTH (2-bit)
+  XtcError m_lastError;
+
+  // Internal helper functions
+  XtcError readHeader();
+  XtcError readPageTable();
+  XtcError readTitle();
+};
+
+}  // namespace xtc