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>

src/activities/reader/XtcReaderActivity.h

@@ -0,0 +1,41 @@
+/**
+ * XtcReaderActivity.h
+ *
+ * XTC ebook reader activity for CrossPoint Reader
+ * Displays pre-rendered XTC pages on e-ink display
+ */
+
+#pragma once
+
+#include <Xtc.h>
+#include <freertos/FreeRTOS.h>
+#include <freertos/semphr.h>
+#include <freertos/task.h>
+
+#include "activities/Activity.h"
+
+class XtcReaderActivity final : public Activity {
+  std::shared_ptr<Xtc> xtc;
+  TaskHandle_t displayTaskHandle = nullptr;
+  SemaphoreHandle_t renderingMutex = nullptr;
+  uint32_t currentPage = 0;
+  int pagesUntilFullRefresh = 0;
+  bool updateRequired = false;
+  const std::function<void()> onGoBack;
+  const std::function<void()> onGoHome;
+
+  static void taskTrampoline(void* param);
+  [[noreturn]] void displayTaskLoop();
+  void renderScreen();
+  void renderPage();
+  void saveProgress() const;
+  void loadProgress();
+
+ public:
+  explicit XtcReaderActivity(GfxRenderer& renderer, InputManager& inputManager, std::unique_ptr<Xtc> xtc,
+                             const std::function<void()>& onGoBack, const std::function<void()>& onGoHome)
+      : Activity("XtcReader", renderer, inputManager), xtc(std::move(xtc)), onGoBack(onGoBack), onGoHome(onGoHome) {}
+  void onEnter() override;
+  void onExit() override;
+  void loop() override;
+};