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.cpp

@@ -253,6 +253,19 @@ bool CrossPointSettings::loadFromBinaryFile() {
 }
 
 float CrossPointSettings::getReaderLineCompression() const {
+  // SD card fonts use same compression as Bookerly (the most neutral values)
+  if (sdFontFamilyName[0] != '\0') {
+    switch (lineSpacing) {
+      case TIGHT:
+        return 0.95f;
+      case NORMAL:
+      default:
+        return 1.0f;
+      case WIDE:
+        return 1.1f;
+    }
+  }
+
   switch (fontFamily) {
     case NOTOSERIF:
     default:
@@ -321,6 +334,13 @@ int CrossPointSettings::getRefreshFrequency() const {
 }
 
 int CrossPointSettings::getReaderFontId() const {
+  // Check SD card font first
+  if (sdFontFamilyName[0] != '\0' && sdFontIdResolver) {
+    int id = sdFontIdResolver(sdFontResolverCtx, sdFontFamilyName, fontSize);
+    if (id != 0) return id;
+    // Fall through to built-in if SD font not found
+  }
+
   switch (fontFamily) {
     case NOTOSERIF:
     default: