feat: add multiboot2 framebuffer and Forth pixel primitives

Co-authored-by: aider (openrouter/moonshotai/kimi-k2.6) <aider@aider.chat>

kernel_entry.S

@@ -8,6 +8,19 @@ multiboot_header:
     .long multiboot_header_end - multiboot_header
     .long -(0xe85250d6 + 0 + (multiboot_header_end - multiboot_header))
 
+    /* Framebuffer tag: type=5, prefer RGB 32 bpp */
+    .align 8
+    .word 5
+    .word 0
+    .long 32
+    .quad 0
+    .long 0
+    .long 0
+    .long 32
+    .byte 1
+    .byte 0
+    .word 0
+
     .align 8
     .word 0
     .word 0
@@ -28,12 +41,17 @@ stack_bottom:
     .space 16384
 stack_top:
 
+.align 8
+mboot_info_ptr:
+    .space 8
+
 .section .text
 .code32
 .global _start
 .type _start, @function
 _start:
     movl $stack_top, %esp
+    movl %ebx, mboot_info_ptr
 
     cld
     movl $__bss_start, %edi
@@ -96,6 +114,7 @@ long_mode_start:
     /* Ensure 16-byte alignment for the System V AMD64 ABI. */
     andq $-16, %rsp
 
+    movq mboot_info_ptr, %rdi
     call kernel_main
     cli
 1: