fix: request text mode in multiboot2 header for VGA output

When using multiboot2, GRUB may set a graphical framebuffer instead
of text mode, causing VGA text writes to 0xB8000 to be invisible.

- Add MULTIBOOT_HEADER_TAG_FRAMEBUFFER to multiboot2 header requesting
  80x25 text mode (depth=0)
- Add 'set gfxpayload=text' to grub.cfg as additional safeguard

This fixes blank display when running under UTM/QEMU with certain
VGA device configurations.

CMakeLists.txt

@@ -38,7 +38,7 @@ add_custom_command(
 add_custom_target(initrd DEPENDS ${INITRD_FILE})
 
 # Create grub.cfg for ISO - includes module2 for the initrd
-file(WRITE ${CMAKE_BINARY_DIR}/isodir/boot/grub/grub.cfg "set timeout=0\nset default=0\nsearch --set=root --file /boot/kernel.bin\nmenuentry \"ClaudeOS\" { multiboot2 /boot/kernel.bin\n  module2 /boot/initrd.cpio }")
+file(WRITE ${CMAKE_BINARY_DIR}/isodir/boot/grub/grub.cfg "set timeout=0\nset default=0\nsearch --set=root --file /boot/kernel.bin\nmenuentry \"ClaudeOS\" {\n  set gfxpayload=text\n  multiboot2 /boot/kernel.bin\n  module2 /boot/initrd.cpio\n}")
 
 
 # ISO Generation

src/boot.S

@@ -26,9 +26,17 @@ multiboot_header:
     /* checksum */
     .long -(MULTIBOOT2_HEADER_MAGIC + MULTIBOOT_ARCHITECTURE_I386 + (multiboot_header_end - multiboot_header))
 
-    /* Tags here */
+    /* Framebuffer tag: request 80x25 EGA text mode */
+    .align 8
+    .short MULTIBOOT_HEADER_TAG_FRAMEBUFFER
+    .short 0         /* flags: required (not optional) */
+    .long 20         /* size of this tag */
+    .long 80         /* width */
+    .long 25         /* height */
+    .long 0          /* depth: 0 = text mode */
 
     /* End tag */
+    .align 8
     .short MULTIBOOT_HEADER_TAG_END
     .short 0
     .long 8