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/SdCardFontSystem.h

@@ -0,0 +1,52 @@
+#pragma once
+
+#include <SdCardFontManager.h>
+#include <SdCardFontRegistry.h>
+
+#include <atomic>
+
+class GfxRenderer;
+
+/// Facade that owns the SD card font registry, manager, and resolver logic.
+/// Hides implementation details behind a single begin() + ensureLoaded() API.
+class SdCardFontSystem {
+ public:
+  SdCardFontSystem() = default;
+  SdCardFontSystem(const SdCardFontSystem&) = delete;
+  SdCardFontSystem& operator=(const SdCardFontSystem&) = delete;
+  /// Discover SD card fonts and load user's saved selection. Call once during setup.
+  void begin(GfxRenderer& renderer);
+
+  /// Ensure the correct SD font family is loaded for the current settings.
+  /// Call before entering the reader or after settings change.
+  /// Also re-discovers if the registry has been marked dirty (e.g. by web upload).
+  void ensureLoaded(GfxRenderer& renderer);
+
+  /// Resolve an SD card font ID from family name + fontSize enum.
+  /// Returns 0 if not found. Used by CrossPointSettings::getReaderFontId().
+  int resolveFontId(const char* familyName, uint8_t fontSizeEnum) const;
+
+  /// Access the registry (e.g. for settings UI to enumerate available fonts).
+  const SdCardFontRegistry& registry() const { return registry_; }
+
+  /// Non-const access to the registry (for FontInstaller).
+  SdCardFontRegistry& registry() { return registry_; }
+
+  /// Mark the registry as needing re-discovery.
+  /// Thread-safe: can be called from the web server task.
+  void markRegistryDirty() { registryDirty_.store(true, std::memory_order_release); }
+
+  /// If the registry is dirty, re-scan the SD card now and clear the flag.
+  /// Used by the web UI so uploaded/deleted fonts appear in the list
+  /// without waiting for the reader activity to run ensureLoaded().
+  void refreshIfDirty() {
+    if (registryDirty_.exchange(false, std::memory_order_acquire)) {
+      registry_.discover();
+    }
+  }
+
+ private:
+  SdCardFontRegistry registry_;
+  SdCardFontManager manager_;
+  std::atomic<bool> registryDirty_{false};
+};