feat: Support for kerning and ligatures (#873)

## Summary

**What is the goal of this PR?**
Improved typesetting, including
[kerning](https://en.wikipedia.org/wiki/Kerning) and
[ligatures](https://en.wikipedia.org/wiki/Ligature_(writing)#Latin_alphabet).

**What changes are included?**
- The script to convert built-in fonts now adds kerning and ligature
information to the generated font headers.
- Epub page layout calculates proper kerning spaces and makes ligature
substitutions according to the selected font.


![3U1B1808](https://github.com/user-attachments/assets/1accb16f-2f1a-41e5-adca-89f1f1348494)

![3U1B1810](https://github.com/user-attachments/assets/2f6bd007-490e-420f-b774-3380b4add7ea)

![3U1B1815](https://github.com/user-attachments/assets/1986bb77-2db0-46e2-a5d6-8315dae9eb19)

## Additional Context

- I am not a typography expert. 
- The implementation has been reworked from the earlier version, so it
is no longer necessary to omit Open Dyslexic, and kerning data now
covers all fonts, styles, and codepoints for which we include bitmap
data.
- Claude Opus 4.6 helped with a lot of this.
- There's an included test epub document with lots of kerning and
ligature examples, shown in the photos.

**_After some time to mature, I think this change is in decent shape to
merge and get people testing._**

After opening this PR I came across #660, which overlaps in adding
ligature support.

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
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? _**YES, Claude Opus 4.6**_

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/EpdFont/EpdFontData.h

@@ -31,6 +31,20 @@ typedef struct {
   uint32_t offset;  ///< Index of the first code point into the glyph array
 } EpdUnicodeInterval;
 
+/// Maps a codepoint to a kerning class ID, sorted by codepoint for binary search.
+/// Class IDs are 1-based; codepoints not in the table have implicit class 0 (no kerning).
+typedef struct {
+  uint16_t codepoint;  ///< Unicode codepoint
+  uint8_t classId;     ///< 1-based kerning class ID
+} __attribute__((packed)) EpdKernClassEntry;
+
+/// Ligature substitution for a specific glyph pair, sorted by `pair` for binary search.
+/// `pair` encodes (leftCodepoint << 16 | rightCodepoint) for single-key lookup.
+typedef struct {
+  uint32_t pair;        ///< Packed codepoint pair (left << 16 | right)
+  uint32_t ligatureCp;  ///< Codepoint of the replacement ligature glyph
+} __attribute__((packed)) EpdLigaturePair;
+
 /// Data stored for FONT AS A WHOLE
 typedef struct {
   const uint8_t* bitmap;                ///< Glyph bitmaps, concatenated
@@ -41,6 +55,15 @@ typedef struct {
   int ascender;                         ///< Maximal height of a glyph above the base line
   int descender;                        ///< Maximal height of a glyph below the base line
   bool is2Bit;
-  const EpdFontGroup* groups;  ///< NULL for uncompressed fonts
-  uint16_t groupCount;         ///< 0 for uncompressed fonts
+  const EpdFontGroup* groups;                 ///< NULL for uncompressed fonts
+  uint16_t groupCount;                        ///< 0 for uncompressed fonts
+  const EpdKernClassEntry* kernLeftClasses;   ///< Sorted left-side class map (nullptr if none)
+  const EpdKernClassEntry* kernRightClasses;  ///< Sorted right-side class map (nullptr if none)
+  const int8_t* kernMatrix;                   ///< Flat leftClassCount x rightClassCount matrix
+  uint16_t kernLeftEntryCount;                ///< Entries in kernLeftClasses
+  uint16_t kernRightEntryCount;               ///< Entries in kernRightClasses
+  uint8_t kernLeftClassCount;                 ///< Number of distinct left classes (matrix rows)
+  uint8_t kernRightClassCount;                ///< Number of distinct right classes (matrix cols)
+  const EpdLigaturePair* ligaturePairs;       ///< Sorted ligature pair table (nullptr if none)
+  uint32_t ligaturePairCount;                 ///< Number of entries in ligaturePairs
 } EpdFontData;