perf: font-compression improvements (#1056)

## Purpose

This PR includes some preparatory changes that are needed for an
upcoming performant CJK font feature. The changes have no impact on
render time and heap allocation for latin text. **Despite this, I think
these changes stand on their own as a better font
compression/decompression implementation.**

## Summary

- Font decompressor rewrite: Replaced the 4-slot LRU group cache with a
two-tier system — a page buffer (glyphs prewarmed before rendering
begins) and a hot-group fallback (last decompressed group retained for
non-prewarmed
  glyphs). 
- Byte-aligned compressed bitmap format: Glyph bitmaps within compressed
groups are now stored row-padded rather than tightly packed before
DEFLATE compression, improving compression ratios by making identical
pixel rows produce
identical byte patterns. Glyphs are compacted back to packed format on
demand at render time. Reduces flash size by 155 KB.
- Page prewarm system: Added `Page::collectText` and
`Page::getDominantStyle` to extract per-style glyph requirements before
rendering, and `GfxRenderer::prewarmFontCache` to pre-decompress only
the groups needed for the dominant style
   — eliminating mid-render decompression for the common case.
- UTF-8 robustness fixes: `utf8NextCodepoint` now validates continuation
bytes and returns a replacement glyph on malformed input;
`ChapterHtmlSlimParser` correctly preserves incomplete multi-byte
sequences across word-buffer flush
  boundaries rather than splitting them.

---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**YES**_ Architecture and
design was done by me, refined a bit by Claude. Code mostly by Claude,
but not entirely.

lib/EpdFont/EpdFontData.h

@@ -50,7 +50,7 @@ typedef struct {
   uint32_t compressedSize;    ///< Compressed DEFLATE stream size
   uint32_t uncompressedSize;  ///< Decompressed size
   uint16_t glyphCount;        ///< Number of glyphs in this group
-  uint16_t firstGlyphIndex;   ///< First glyph index in the global glyph array
+  uint32_t firstGlyphIndex;   ///< First glyph index in the global glyph array
 } EpdFontGroup;
 
 /// Glyph interval structure
@@ -86,6 +86,7 @@ typedef struct {
   bool is2Bit;
   const EpdFontGroup* groups;                 ///< NULL for uncompressed fonts
   uint16_t groupCount;                        ///< 0 for uncompressed fonts
+  const uint16_t* glyphToGroup;               ///< Per-glyph group ID (nullptr for contiguous-group fonts)
   const EpdKernClassEntry* kernLeftClasses;   ///< Sorted left-side class map (nullptr if none)
   const EpdKernClassEntry* kernRightClasses;  ///< Sorted right-side class map (nullptr if none)
   const int8_t* kernMatrix;              ///< Flat leftClassCount x rightClassCount matrix, 4.4 fixed-point in pixels