fix: correct multiboot2 framebuffer tag layout

Co-authored-by: aider (openrouter/anthropic/claude-opus-4.7) <aider@aider.chat>

kernel_entry.S

@@ -8,18 +8,18 @@ multiboot_header:
     .long multiboot_header_end - multiboot_header
     .long -(0xe85250d6 + 0 + (multiboot_header_end - multiboot_header))
 
-    /* Framebuffer tag: type=5, prefer RGB 32 bpp */
+    /* Framebuffer tag: type=5, size=20, width=height=0 (any), depth=32.
+     * Layout per multiboot2 spec:
+     *   u16 type, u16 flags, u32 size, u32 width, u32 height, u32 depth
+     * Padded to 8-byte alignment. */
     .align 8
     .word 5
     .word 0
-    .long 32
-    .quad 0
+    .long 20
     .long 0
     .long 0
     .long 32
-    .byte 1
-    .byte 0
-    .word 0
+    .long 0       /* padding to 8-byte alignment */
 
     .align 8
     .word 0