display: a 64-bit framebuffer address, no mirroring onto itself

Three fixes for real machines that broke at any resolution but 800x600:

- kmain() took the loader's framebuffer address as uint32, and
  vga_fb_init() re-published the scanout from that and the 16-bit VBE
  block. An aperture above 4 GB moved the console onto the wrong memory
  right after the vga: line. Both now use the loader's 64-bit display
  block, mapped with ioremap().
- The loader mirrored onto the console splitter's GOP, which forwards to
  the primary itself, and onto GOPs that share the primary's aperture.
  After a mode change that rewrote the screen over itself with another
  mode's stride. Handles without a device path and overlapping apertures
  are skipped, and a mode change re-matches the remaining mirrors.
- A mode whose reported size, stride or aperture does not add up after
  SetMode is refused, and the loader returns to the previous one.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

boot/uefi/efi.h

@@ -218,6 +218,15 @@ typedef struct {
     void     *VendorTable;
 } EFI_CONFIGURATION_TABLE;
 
+// --- device path ----------------------------------------------------------
+//
+// Only ever asked whether a handle *has* one: a protocol installed by a real
+// device's driver sits on a handle with a path to that device, and a virtual
+// one -- the console splitter's GOP -- sits on a handle with none.
+
+#define EFI_DEVICE_PATH_PROTOCOL_GUID \
+    { 0x09576E91, 0x6D3F, 0x11D2, { 0x8E, 0x39, 0x00, 0xA0, 0xC9, 0x69, 0x72, 0x3B } }
+
 // --- loaded image ---------------------------------------------------------
 
 #define EFI_LOADED_IMAGE_PROTOCOL_GUID \