Add FreeInkBook EPUB engine migration guide

Document migration plan from CrossPoint's lib/Epub to FreeInkBook, a memory-efficient clean-room EPUB engine. Covers advantages (O(paragraph+page) memory via SAX parsing, arena allocation, host-testable, full UAX#14 typography, runtime TTF/OTF support), 9-step migration strategy, ESP32-C3 constraints, and integration approach preserving existing UI layer while replacing parsing/layout stack.

lib/EpdFont/EpdFont.cpp

@@ -154,6 +154,21 @@ uint32_t EpdFont::applyLigatures(uint32_t cp, const char*& text) const {
   return cp;
 }
 
+bool EpdFont::hasGlyph(const uint32_t cp) const {
+  const int count = data->intervalCount;
+  if (count > 0) {
+    const EpdUnicodeInterval* intervals = data->intervals;
+    const auto it =
+        std::upper_bound(intervals, intervals + count, cp,
+                         [](uint32_t value, const EpdUnicodeInterval& interval) { return value < interval.first; });
+    if (it != intervals && cp <= (it - 1)->last) return true;
+  }
+  if (data->glyphMissHandler) {
+    return data->glyphMissHandler(data->glyphMissCtx, cp) != nullptr;
+  }
+  return false;
+}
+
 const EpdGlyph* EpdFont::getGlyph(const uint32_t cp) const {
   const int count = data->intervalCount;
   if (count == 0 && !data->glyphMissHandler) return nullptr;

lib/EpdFont/EpdFont.h

@@ -12,6 +12,12 @@ class EpdFont {
 
   const EpdGlyph* getGlyph(uint32_t cp) const;
 
+  /// True when the font actually provides `cp` (interval table or on-demand
+  /// SD load) — unlike getGlyph(), never satisfied by the replacement glyph.
+  /// Used by layout shaping to decide whether a substitution (ligature,
+  /// Arabic presentation form) can really be drawn.
+  bool hasGlyph(uint32_t cp) const;
+
   /// Returns the kerning adjustment (4.4 fixed-point in pixels) between two codepoints.
   /// Returns 0 if no kerning data exists for the pair.
   int8_t getKerning(uint32_t leftCp, uint32_t rightCp) const;

lib/EpdFont/EpdFontFamily.cpp

@@ -28,10 +28,18 @@ const EpdGlyph* EpdFontFamily::getGlyph(const uint32_t cp, const Style style) co
   return getFont(style)->getGlyph(cp);
 }
 
+bool EpdFontFamily::hasGlyph(const uint32_t cp, const Style style) const {
+  return getFont(style)->hasGlyph(cp);
+}
+
 int8_t EpdFontFamily::getKerning(const uint32_t leftCp, const uint32_t rightCp, const Style style) const {
   return getFont(style)->getKerning(leftCp, rightCp);
 }
 
+uint32_t EpdFontFamily::getLigature(const uint32_t leftCp, const uint32_t rightCp, const Style style) const {
+  return getFont(style)->getLigature(leftCp, rightCp);
+}
+
 uint32_t EpdFontFamily::applyLigatures(const uint32_t cp, const char*& text, const Style style) const {
   return getFont(style)->applyLigatures(cp, text);
 }

lib/EpdFont/EpdFontFamily.h

@@ -26,7 +26,9 @@ class EpdFontFamily {
   void getTextDimensions(const char* string, int* w, int* h, Style style = REGULAR) const;
   const EpdFontData* getData(Style style = REGULAR) const;
   const EpdGlyph* getGlyph(uint32_t cp, Style style = REGULAR) const;
+  bool hasGlyph(uint32_t cp, Style style = REGULAR) const;
   int8_t getKerning(uint32_t leftCp, uint32_t rightCp, Style style = REGULAR) const;
+  uint32_t getLigature(uint32_t leftCp, uint32_t rightCp, Style style = REGULAR) const;
   uint32_t applyLigatures(uint32_t cp, const char*& text, Style style = REGULAR) const;
   static constexpr bool hasTextDecoration(const Style style) {
     return (static_cast<uint8_t>(style) & TEXT_DECORATION_MASK) != 0;