Convert BDF fonts to Unicode-capable font system

[pbalduino]

Summary

Convert existing BDF font files to support Unicode codepoints and implement a Unicode-aware font rendering system.

Description

Replace the current ASCII-only font system with a Unicode-capable system that can load and render glyphs from the existing Terminus BDF font files.

Current State

• Font system supports only 256 ASCII characters
• Uses hardcoded glyph definitions in src/kernel/fonts.c
• BDF Unicode font files exist in fonts/ directory but are unused

Tasks

• Create BDF font parser to load fonts/ter-u16*.bdf files
• Implement Unicode codepoint to glyph mapping system
• Replace font_glyph(uint8_t ascii) with font_glyph(uint32_t codepoint)
• Add fallback glyph handling for unsupported codepoints
• Implement font caching for performance
• Support multiple font sizes/styles
• Add memory-efficient storage for large Unicode font sets

Implementation Details

Update include/kernel/fonts.h:

typedef struct {
  uint32_t codepoint;
  uint8_t width;
  uint8_t height;
  uint8_t *bitmap;
} unicode_glyph_t;

typedef struct {
  size_t glyph_count;
  unicode_glyph_t *glyphs;
  // Hash table or tree for fast codepoint lookup
} unicode_font_t;

Files to Modify

• src/kernel/fonts.c - Font loading and glyph lookup
• include/kernel/fonts.h - Font data structures
• src/kernel/framebuffer.c - Update glyph rendering calls

Success Criteria

• Can render basic Latin characters (U+0000-U+007F)
• Can render extended Latin characters (U+0080-U+00FF)
• Fallback glyph displays for unsupported characters
• Performance acceptable for console rendering
• Memory usage reasonable for embedded system

Dependencies

• Depends on: Implement UTF-8 decoding and encoding utilities #127 (UTF-8 utilities)

Priority

High - Required for Unicode text display