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>

lib/EpdFont/EpdFont.cpp

@@ -153,24 +153,32 @@ uint32_t EpdFont::applyLigatures(uint32_t cp, const char*& text) const {
 
 const EpdGlyph* EpdFont::getGlyph(const uint32_t cp) const {
   const int count = data->intervalCount;
-  if (count == 0) return nullptr;
+  if (count == 0 && !data->glyphMissHandler) return nullptr;
 
-  const EpdUnicodeInterval* intervals = data->intervals;
-  const auto* end = intervals + count;
+  if (count > 0) {
+    const EpdUnicodeInterval* intervals = data->intervals;
+    const auto* end = intervals + count;
 
-  // upper_bound: range lookup. Finds the first interval with first > cp, so the
-  // interval just before it is the last one with first <= cp. That's the only
-  // candidate that could contain cp. Then we verify cp <= candidate.last.
-  const auto it = std::upper_bound(
-      intervals, end, cp, [](uint32_t value, const EpdUnicodeInterval& interval) { return value < interval.first; });
+    // upper_bound: range lookup. Finds the first interval with first > cp, so the
+    // interval just before it is the last one with first <= cp. That's the only
+    // candidate that could contain cp. Then we verify cp <= candidate.last.
+    const auto it = std::upper_bound(
+        intervals, end, cp, [](uint32_t value, const EpdUnicodeInterval& interval) { return value < interval.first; });
 
-  if (it != intervals) {
-    const auto& interval = *(it - 1);
-    if (cp <= interval.last) {
-      return &data->glyph[interval.offset + (cp - interval.first)];
+    if (it != intervals) {
+      const auto& interval = *(it - 1);
+      if (cp <= interval.last) {
+        return &data->glyph[interval.offset + (cp - interval.first)];
+      }
     }
   }
 
+  // Codepoint not in interval table — try on-demand loading (SD card fonts).
+  if (data->glyphMissHandler) {
+    const EpdGlyph* loaded = data->glyphMissHandler(data->glyphMissCtx, cp);
+    if (loaded) return loaded;
+  }
+
   if (cp != REPLACEMENT_GLYPH) {
     return getGlyph(REPLACEMENT_GLYPH);
   }