feat: graphical framebuffer support for VGA driver

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.

src/framebuffer.h

@@ -0,0 +1,44 @@
+/**
+ * @file framebuffer.h
+ * @brief Framebuffer information from the bootloader.
+ *
+ * Stores the display mode and framebuffer address provided by GRUB
+ * via the multiboot2 framebuffer tag. The VGA driver uses this to
+ * decide between text-mode writes (0xB8000) and pixel rendering.
+ */
+
+#ifndef FRAMEBUFFER_H
+#define FRAMEBUFFER_H
+
+#include <stdint.h>
+
+/** Framebuffer types (matches multiboot2 definitions). */
+#define FB_TYPE_INDEXED  0
+#define FB_TYPE_RGB      1
+#define FB_TYPE_EGA_TEXT 2
+
+/**
+ * Framebuffer information structure.
+ * Populated during boot from the multiboot2 framebuffer tag.
+ */
+typedef struct {
+    uint32_t addr;         /**< Physical address of the framebuffer. */
+    uint32_t pitch;        /**< Bytes per scanline. */
+    uint32_t width;        /**< Width in pixels (or columns for text). */
+    uint32_t height;       /**< Height in pixels (or rows for text). */
+    uint8_t  bpp;          /**< Bits per pixel. */
+    uint8_t  type;         /**< FB_TYPE_RGB, FB_TYPE_EGA_TEXT, etc. */
+
+    /* RGB field positions (only valid when type == FB_TYPE_RGB). */
+    uint8_t  red_pos;
+    uint8_t  red_size;
+    uint8_t  green_pos;
+    uint8_t  green_size;
+    uint8_t  blue_pos;
+    uint8_t  blue_size;
+} framebuffer_info_t;
+
+/** Global framebuffer info, filled by kernel_main. */
+extern framebuffer_info_t fb_info;
+
+#endif /* FRAMEBUFFER_H */