When GRUB is configured with multiboot2, it may provide a graphical
(RGB) framebuffer instead of legacy VGA text mode. This happens on
UTM/QEMU on macOS and other configurations where the bootloader
switches to a pixel-based display.
- Parse multiboot2 framebuffer tag in kernel_main to detect display mode
- Identity-map the framebuffer address (often at 0xFD000000+) after
paging is enabled, with write-through caching
- Add framebuffer.h describing the boot-time display info
- Embed 8x16 VGA bitmap font (font8x16.h) for pixel-mode rendering
- Rewrite VGA driver to support both text mode and pixel mode:
- Text mode: unchanged behavior writing to 0xB8000
- Pixel mode: renders characters using the bitmap font to the
GRUB-provided framebuffer, with proper VGA color palette mapping
- Auto-detects mode from fb_info at init time
- Multiboot2 header now requests text mode via framebuffer tag, but
gracefully falls back to pixel rendering if GRUB provides RGB
- Reverted grub.cfg gfxpayload=text (caused display issues on UTM)
Tested: boots in both text mode and graphical framebuffer mode.
