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: #127 (UTF-8 utilities)
• Depends on: #128 (Unicode font system)

Priority

High - Core Unicode rendering functionality