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/GfxRenderer/GfxRenderer.h

@@ -4,6 +4,7 @@
 #include <HalDisplay.h>
 
 class FontCacheManager;
+class SdCardFont;
 
 #include <cstring>
 #include <map>
@@ -42,6 +43,11 @@ class GfxRenderer {
   uint32_t frameBufferSize = HalDisplay::BUFFER_SIZE;
   std::vector<uint8_t*> bwBufferChunks;
   std::map<int, EpdFontFamily> fontMap;
+  // Mutable because ensureSdCardFontReady() is const (called from layout code
+  // that holds a const GfxRenderer&) but triggers SD card reads and heap
+  // allocation inside the SdCardFont objects. Same pragmatic compromise as
+  // fontCacheManager_ below.
+  mutable std::map<int, SdCardFont*> sdCardFonts_;
 
   // Mutable because drawText() is const but needs to delegate scan-mode
   // recording to the (non-const) FontCacheManager. Same pragmatic compromise
@@ -69,9 +75,25 @@ class GfxRenderer {
   // Setup
   void begin();  // must be called right after display.begin()
   void insertFont(int fontId, EpdFontFamily font);
+  // Clears both the flash-font map and any SD-font registration for fontId.
+  // Coupled to avoid dangling SdCardFont* in sdCardFonts_ when callers free
+  // the underlying SdCardFont and forget the SD-side unregister.
+  void removeFont(int fontId) {
+    fontMap.erase(fontId);
+    sdCardFonts_.erase(fontId);
+  }
   void setFontCacheManager(FontCacheManager* m) { fontCacheManager_ = m; }
   FontCacheManager* getFontCacheManager() const { return fontCacheManager_; }
   const std::map<int, EpdFontFamily>& getFontMap() const { return fontMap; }
+  void registerSdCardFont(int fontId, SdCardFont* font) { sdCardFonts_[fontId] = font; }
+  void unregisterSdCardFont(int fontId) { removeFont(fontId); }
+  void clearSdCardFonts() { sdCardFonts_.clear(); }
+  const std::map<int, SdCardFont*>& getSdCardFonts() const { return sdCardFonts_; }
+  bool isSdCardFont(int fontId) const { return sdCardFonts_.count(fontId) > 0; }
+  // Ensure SD card font glyph data is loaded for the given text. Called from layout code
+  // (which holds a const GfxRenderer&) before measuring word widths. Safe to call on non-SD fonts (no-op).
+  // styleMask: bitmask of styles to prepare (bit 0=regular, 1=bold, 2=italic, 3=bold-italic).
+  void ensureSdCardFontReady(int fontId, const char* utf8Text, uint8_t styleMask = 0x0F) const;
 
   // Orientation control (affects logical width/height and coordinate transforms)
   void setOrientation(const Orientation o) { orientation = o; }