feat: add multiboot2 framebuffer and Forth pixel primitives

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

platform_linux.c

@@ -107,6 +107,13 @@ void forth_printf(const char* fmt, ...) {
     va_end(ap);
 }
 
+void do_fb_addr(Word* w) { (void)w; data_push(0); }
+void do_fb_width(Word* w) { (void)w; data_push(0); }
+void do_fb_height(Word* w) { (void)w; data_push(0); }
+void do_fb_pitch(Word* w) { (void)w; data_push(0); }
+void do_fb_bpp(Word* w) { (void)w; data_push(0); }
+void do_fb_plot(Word* w) { (void)w; data_pop(); data_pop(); data_pop(); }
+
 void _start(void) {
     forth_run();
     syscall1(60, 0);