feat: add SD card font support with on-device download and web management

Add a complete SD card font subsystem that enables users to install and
use custom fonts beyond the three built-in families. This combines the
back-end firmware support (#1327) with the font configuration, build
pipeline, CI distribution, and user-facing management UI (#1392).

Core font system:
- Custom .cpfont binary format (v4) with multi-style support (regular,
  bold, italic, bold-italic) packed into a single file per size
- On-demand glyph loading from SD card with two-pass prewarm rendering
  to bulk-read glyphs per page, achieving near-flash performance for
  Latin text (~697ms vs ~681ms) and viable CJK rendering (~32% slower)
- Persistent advance cache for layout measurement without SD I/O
- Overflow ring buffer for glyph cache misses during rendering
- Memory-conscious design: only advance tables kept in RAM; glyph
  bitmaps, kern tables, and ligatures loaded on demand from SD

Font management:
- On-device WiFi download from GitHub Releases with manifest-based
  discovery, install/update detection, and progress UI
- Web interface font upload, listing, and deletion via /fonts page
- Manual SD card copy to /fonts/ or /.fonts/ directories
- Font selection integrated into Settings > Reader > Font Family

Build pipeline:
- Declarative YAML config (sd-fonts.yaml) as single source of truth
  for the 17-family font library (serif, sans, mono, accessibility)
- Python converter (fontconvert_sdcard.py) for TTF/OTF to .cpfont with
  FreeType rasterization, class-based kerning, and ligature extraction
- Parallel build orchestrator with variable font instance extraction
- CI workflow publishing versioned + stable releases to a dedicated
  crosspoint-fonts repository with auto-incrementing revision tags
- Centralized version constants (cpfont_version.py) shared across
  build tooling and CI, with firmware headers as manual sync points

Additional fixes:
- CJK characters no longer get hyphens inserted at line breaks
- Advance table eliminates 30+ second stalls during CJK section
  indexing for paragraphs with >512 unique codepoints

Closes #930

Co-authored-by: Zach Nelson <zach@zdnelson.com>
Co-authored-by: Justin <itsthisjustin@users.noreply.github.com>
Co-authored-by: jpirnay <jens@pirnay.com>
Co-authored-by: mcrosson <kemonine@kemonine.info>

src/CrossPointSettings.h

@@ -97,8 +97,9 @@ class CrossPointSettings {
   // Swapped: Next, Previous
   enum SIDE_BUTTON_LAYOUT { PREV_NEXT = 0, NEXT_PREV = 1, SIDE_BUTTON_LAYOUT_COUNT };
 
-  // Font family options
+  // Font family options (built-in fonts only; SD card fonts use sdFontFamilyName)
   enum FONT_FAMILY { NOTOSERIF = 0, NOTOSANS = 1, OPENDYSLEXIC = 2, FONT_FAMILY_COUNT };
+  static constexpr uint8_t BUILTIN_FONT_COUNT = FONT_FAMILY_COUNT;
   // Font size options
   enum FONT_SIZE { SMALL = 0, MEDIUM = 1, LARGE = 2, EXTRA_LARGE = 3, FONT_SIZE_COUNT };
   enum LINE_COMPRESSION { TIGHT = 0, NORMAL = 1, WIDE = 2, LINE_COMPRESSION_COUNT };
@@ -212,6 +213,8 @@ class CrossPointSettings {
   uint8_t fadingFix = 0;
   // Use book's embedded CSS styles for EPUB rendering (1 = enabled, 0 = disabled)
   uint8_t embeddedStyle = 1;
+  // SD card font family name (empty = use built-in fontFamily)
+  char sdFontFamilyName[32] = "";
   // Show hidden files/directories (starting with '.') in the file browser (0 = hidden, 1 = show)
   uint8_t showHiddenFiles = 0;
   // Image rendering mode in EPUB reader
@@ -226,6 +229,12 @@ class CrossPointSettings {
   // Get singleton instance
   static CrossPointSettings& getInstance() { return instance; }
 
+  // Callback to resolve SD card font IDs. Set by SdCardFontSystem::begin().
+  // Returns font ID or 0 if not found.
+  using SdFontIdResolver = int (*)(void* ctx, const char* familyName, uint8_t fontSize);
+  SdFontIdResolver sdFontIdResolver = nullptr;
+  void* sdFontResolverCtx = nullptr;
+
   uint16_t getPowerButtonDuration() const {
     return (shortPwrBtn == CrossPointSettings::SHORT_PWRBTN::SLEEP) ? 10 : 400;
   }