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/Epub/Epub/ParsedText.cpp

@@ -100,6 +100,37 @@ void ParsedText::layoutAndExtractLines(const GfxRenderer& renderer, const int fo
   // Apply fixed transforms before any per-line layout work.
   applyParagraphIndent();
 
+  // Ensure SD card font glyph metrics are loaded before measuring word widths.
+  // For flash-based fonts isSdCardFont() returns false and this block is skipped
+  // entirely — no heap allocation. For SD card fonts this reads glyph metadata
+  // (advanceX only, no bitmaps) for all unique codepoints in this paragraph so
+  // that calculateWordWidths() can measure text without on-demand SD I/O.
+  if (renderer.isSdCardFont(fontId)) {
+    // Reserve upfront so the joined text allocates exactly once. Without this,
+    // paragraphs with many words trigger a chain of vector-like reallocations
+    // inside std::string during layout — visible in prewarm timings for SD fonts.
+    size_t totalSize = hyphenationEnabled ? 1 : 0;
+    if (!words.empty()) totalSize += words.size() - 1;  // inter-word spaces
+    for (const auto& w : words) totalSize += w.size();
+    std::string allText;
+    allText.reserve(totalSize);
+    for (size_t i = 0; i < words.size(); i++) {
+      if (i > 0) allText += ' ';
+      allText += words[i];
+    }
+    if (hyphenationEnabled) allText += '-';
+
+    // Style mask: only ask the SD font to load advances for styles actually
+    // used in this paragraph. Style index is the low two bits (regular/bold/
+    // italic/bold-italic); the underline bit is irrelevant to advance metrics.
+    uint8_t styleMask = 0;
+    for (auto s : wordStyles) {
+      styleMask |= static_cast<uint8_t>(1u << (static_cast<uint8_t>(s) & 0x03));
+    }
+    if (styleMask == 0) styleMask = 0x01;  // defensive: regular only
+    renderer.ensureSdCardFontReady(fontId, allText.c_str(), styleMask);
+  }
+
   const int pageWidth = viewportWidth;
   auto wordWidths = calculateWordWidths(renderer, fontId);
 

lib/Epub/Epub/hyphenation/Hyphenator.cpp

@@ -1,5 +1,7 @@
 #include "Hyphenator.h"
 
+#include <Utf8.h>
+
 #include <algorithm>
 #include <cassert>
 #include <vector>
@@ -256,7 +258,16 @@ std::vector<Hyphenator::BreakInfo> Hyphenator::breakOffsets(const std::string& w
   std::vector<Hyphenator::BreakInfo> breaks;
   breaks.reserve(indexes.size());
   for (const size_t idx : indexes) {
-    breaks.push_back({byteOffsetForIndex(cps, idx), true});
+    // CJK characters can break without inserting a visible hyphen.
+    // Check the codepoint at the break position: if it's a CJK character,
+    // no hyphen is needed since CJK scripts don't use hyphenation.
+    bool needsHyphen = true;
+    if (idx < cps.size() && utf8IsCjkBreakable(cps[idx].value)) {
+      needsHyphen = false;
+    } else if (idx > 0 && utf8IsCjkBreakable(cps[idx - 1].value)) {
+      needsHyphen = false;
+    }
+    breaks.push_back({byteOffsetForIndex(cps, idx), needsHyphen});
   }
 
   return breaks;