Implement Unicode text rendering in framebuffer

[pbalduino]

Summary

Update the framebuffer text rendering system to properly handle UTF-8 encoded Unicode text.

Description

Modify the framebuffer console to decode UTF-8 byte sequences and render Unicode characters using the new Unicode-capable font system.

Current State

• fb_putchar() treats all input as single-byte ASCII
• Text rendering assumes 1 byte = 1 character
• Console buffer stores only ASCII characters

Tasks

• Update fb_putchar() to handle UTF-8 byte sequences
• Implement UTF-8 state machine for multi-byte character assembly
• Update console buffer to store Unicode codepoints
• Handle variable-width character rendering
• Update cursor positioning for Unicode text
• Add support for combining characters (optional)
• Handle right-to-left text (optional, future enhancement)

Implementation Details

Update src/kernel/framebuffer.c:

• Modify console_cell_t to store uint32_t codepoint instead of char ch
• Add UTF-8 decoder state to handle incomplete sequences
• Update write_char() to call new Unicode font system
• Handle width calculation for characters (most are single-width)

Key Functions to Update

• fb_putchar(int c) - Add UTF-8 decoding
• write_char(int c) - Change to write_codepoint(uint32_t cp)
• draw_cell() - Use Unicode glyph lookup
• Console buffer management functions

Success Criteria

• Can display UTF-8 encoded text correctly
• Handles incomplete UTF-8 sequences gracefully
• Cursor positioning works correctly with Unicode text
• Performance acceptable for interactive use
• Backward compatible with existing ASCII text

Dependencies

• Depends on: Implement UTF-8 decoding and encoding utilities #127 (UTF-8 utilities)
• Depends on: Convert BDF fonts to Unicode-capable font system #128 (Unicode font system)

Priority

High - Core Unicode rendering functionality