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/activities/reader/EpubReaderActivity.cpp

@@ -582,6 +582,8 @@ void EpubReaderActivity::render(RenderLock&& lock) {
                                   SETTINGS.imageRendering)) {
       LOG_DBG("ERS", "Cache not found, building...");
 
+      GUI.drawPopup(renderer, tr(STR_INDEXING));
+
       const auto popupFn = [this]() { GUI.drawPopup(renderer, tr(STR_INDEXING)); };
 
       if (!section->createSectionFile(SETTINGS.getReaderFontId(), SETTINGS.getReaderLineCompression(),
@@ -745,27 +747,19 @@ void EpubReaderActivity::renderContents(std::unique_ptr<Page> page, const int or
                                         const int orientedMarginRight, const int orientedMarginBottom,
                                         const int orientedMarginLeft) {
   const auto t0 = millis();
-  auto* fcm = renderer.getFontCacheManager();
-  fcm->resetStats();
 
   // Font prewarm: scan pass accumulates text, then prewarm, then real render
-  const uint32_t heapBefore = esp_get_free_heap_size();
+  auto* fcm = renderer.getFontCacheManager();
   auto scope = fcm->createPrewarmScope();
   page->render(renderer, SETTINGS.getReaderFontId(), orientedMarginLeft, orientedMarginTop);  // scan pass
   scope.endScanAndPrewarm();
-  const uint32_t heapAfter = esp_get_free_heap_size();
-  fcm->logStats("prewarm");
   const auto tPrewarm = millis();
 
-  LOG_DBG("ERS", "Heap: before=%lu after=%lu delta=%ld", heapBefore, heapAfter,
-          (int32_t)heapAfter - (int32_t)heapBefore);
-
   // Force special handling for pages with images when anti-aliasing is on
   bool imagePageWithAA = page->hasImages() && SETTINGS.textAntiAliasing;
 
   page->render(renderer, SETTINGS.getReaderFontId(), orientedMarginLeft, orientedMarginTop);
   renderStatusBar();
-  fcm->logStats("bw_render");
   const auto tBwRender = millis();
 
   if (imagePageWithAA) {
@@ -816,8 +810,6 @@ void EpubReaderActivity::renderContents(std::unique_ptr<Page> page, const int or
     renderer.displayGrayBuffer();
     const auto tGrayDisplay = millis();
     renderer.setRenderMode(GfxRenderer::BW);
-    fcm->logStats("gray");
-
     // restore the bw data
     renderer.restoreBwBuffer();
     const auto tBwRestore = millis();