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>

docs/sd-card-fonts.md

@@ -0,0 +1,94 @@
+# SD Card Fonts
+
+CrossPoint supports loading additional fonts from the SD card, including fonts
+with extended Unicode coverage (CJK, Cyrillic, Greek, etc.).
+
+## Installing Fonts
+
+There are three ways to install fonts:
+
+### Option 1: Download from device (recommended)
+
+1. Connect your CrossPoint reader to WiFi
+2. Go to **Settings > System > Download Fonts**
+3. Browse available font families and tap to download
+4. Downloaded fonts appear immediately in **Settings > Reader > Font Family**
+
+### Option 2: Upload via web browser
+
+1. Connect your CrossPoint reader to WiFi
+2. Open the web interface in your browser (shown on the WiFi screen)
+3. Navigate to the **Fonts** tab
+4. Upload `.cpfont` files using the upload form
+
+### Option 3: Manual SD card copy
+
+1. Download font files from the
+   [Releases page](https://github.com/crosspoint-reader/crosspoint-reader/releases/tag/sd-fonts)
+2. Copy font family folders to `/.crosspoint/fonts/` on your SD card:
+
+       SD Card Root/
+       └── .crosspoint/
+           └── fonts/
+               ├── Bookerly-SD/
+               │   ├── Bookerly-SD_12.cpfont
+               │   ├── Bookerly-SD_14.cpfont
+               │   ├── Bookerly-SD_16.cpfont
+               │   └── Bookerly-SD_18.cpfont
+               └── ...
+
+3. Insert the SD card and power on your CrossPoint reader
+
+## Available Pre-Built Fonts
+
+| Font | Best For | Languages |
+|------|----------|-----------|
+| Bookerly-SD | General reading | English, Western European |
+| NotoSansExtended | Multi-script reading | European, Greek, Cyrillic, Georgian, Armenian, Ethiopic |
+| NotoSansCJK | Chinese/Japanese/Korean | CJK + ASCII |
+
+## Converting Custom Fonts
+
+To convert your own TrueType/OpenType fonts:
+
+### Prerequisites
+
+    pip install freetype-py fonttools
+
+### Single font (one style)
+
+    python3 lib/EpdFont/scripts/fontconvert_sdcard.py \
+      MyFont-Regular.ttf \
+      --intervals latin-ext \
+      --sizes 12,14,16,18 \
+      --style regular \
+      --name MyFont \
+      --output-dir ./MyFont/
+
+### Multi-style font
+
+    python3 lib/EpdFont/scripts/fontconvert_sdcard.py \
+      --regular MyFont-Regular.ttf \
+      --bold MyFont-Bold.ttf \
+      --italic MyFont-Italic.ttf \
+      --bolditalic MyFont-BoldItalic.ttf \
+      --intervals latin-ext \
+      --sizes 12,14,16,18 \
+      --name MyFont \
+      --output-dir ./MyFont/
+
+### Available Unicode interval presets
+
+| Preset | Coverage |
+|--------|----------|
+| `ascii` | U+0020-U+007E (Basic Latin) |
+| `latin-ext` | European languages (Latin + Extended-A/B) |
+| `greek` | Greek + Extended Greek |
+| `cyrillic` | Cyrillic + Supplement |
+| `cjk` | CJK Unified Ideographs + Hiragana + Katakana + Fullwidth |
+| `hangul` | Korean Hangul syllables |
+| `builtin` | Matches built-in Bookerly coverage exactly |
+
+Combine presets with commas: `--intervals latin-ext,greek,cyrillic`
+
+Install custom fonts via WiFi upload or manual SD card copy.