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/SdCardFontManager.cpp

@@ -0,0 +1,98 @@
+#include "SdCardFontManager.h"
+
+#include <EpdFontFamily.h>
+#include <GfxRenderer.h>
+#include <Logging.h>
+#include <SdCardFont.h>
+#include <SdCardFontRegistry.h>
+
+SdCardFontManager::~SdCardFontManager() {
+  for (auto& lf : loaded_) {
+    delete lf.font;
+  }
+}
+
+// FNV-1a continuation: seeds with contentHash, then hashes family name + point size.
+// Produces a deterministic ID that is stable across load/unload cycles and reboots,
+// and changes when font content changes (different header/TOC = different contentHash).
+int SdCardFontManager::computeFontId(uint32_t contentHash, const char* familyName, uint8_t pointSize) {
+  static constexpr uint32_t FNV_PRIME = 16777619u;
+  uint32_t hash = contentHash;
+  while (*familyName) {
+    hash ^= static_cast<uint8_t>(*familyName++);
+    hash *= FNV_PRIME;
+  }
+  hash ^= pointSize;
+  hash *= FNV_PRIME;
+  int id = static_cast<int>(hash);
+  return id != 0 ? id : 1;  // 0 is reserved as "not found" sentinel
+}
+
+bool SdCardFontManager::loadFamily(const SdCardFontFamilyInfo& family, GfxRenderer& renderer, uint8_t fontSizeEnum) {
+  // Unload any previously loaded family first
+  if (!loadedFamilyName_.empty()) {
+    unloadAll(renderer);
+  }
+
+  // Select by ordinal position: sort available sizes, then map the font size
+  // enum (SMALL=0 .. EXTRA_LARGE=3) to the corresponding slot. When the
+  // family has fewer sizes than 4, clamp to the last available size.
+  auto sizes = family.availableSizes();
+  if (sizes.empty()) {
+    LOG_ERR("SDMGR", "Family %s has no files to load", family.name.c_str());
+    return false;
+  }
+
+  uint8_t idx = fontSizeEnum;
+  if (idx >= sizes.size()) idx = sizes.size() - 1;
+  const SdCardFontFileInfo* selected = family.findFile(sizes[idx]);
+
+  auto* font = new (std::nothrow) SdCardFont();
+  if (!font) {
+    LOG_ERR("SDMGR", "Failed to allocate SdCardFont for %s", selected->path.c_str());
+    return false;
+  }
+
+  if (!font->load(selected->path.c_str())) {
+    LOG_ERR("SDMGR", "Failed to load %s", selected->path.c_str());
+    delete font;
+    return false;
+  }
+
+  int fontId = computeFontId(font->contentHash(), family.name.c_str(), selected->pointSize);
+  // Guard against collision with built-in font IDs (astronomically unlikely
+  // with FNV-1a hashes, but provides a safety net)
+  if (renderer.getFontMap().count(fontId) != 0) {
+    LOG_ERR("SDMGR", "Font ID %d collides with existing font, skipping %s", fontId, selected->path.c_str());
+    delete font;
+    return false;
+  }
+  renderer.registerSdCardFont(fontId, font);
+  loaded_.push_back({font, fontId, selected->pointSize});
+
+  LOG_DBG("SDMGR", "Loaded %s size=%u id=%d styles=%u (sizeEnum=%u)", selected->path.c_str(), selected->pointSize,
+          fontId, font->styleCount(), fontSizeEnum);
+
+  EpdFontFamily fontFamily(font->getEpdFont(0), font->getEpdFont(1), font->getEpdFont(2), font->getEpdFont(3));
+  renderer.insertFont(fontId, fontFamily);
+
+  loadedFamilyName_ = family.name;
+  loadedPointSize_ = selected->pointSize;
+  return true;
+}
+
+void SdCardFontManager::unloadAll(GfxRenderer& renderer) {
+  renderer.clearSdCardFonts();
+  for (auto& lf : loaded_) {
+    renderer.removeFont(lf.fontId);
+    delete lf.font;
+  }
+  loaded_.clear();
+  loadedFamilyName_.clear();
+  loadedPointSize_ = 0;
+}
+
+int SdCardFontManager::getFontId(const std::string& familyName) const {
+  if (familyName != loadedFamilyName_ || loaded_.empty()) return 0;
+  return loaded_.front().fontId;
+}