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

kernel/core/main.c

@@ -42,8 +42,13 @@ int boot_debug;
 // Bootstrap processor starts running C code here.
 // Allocate a real stack and switch to it, first
 // doing some setup required for memory allocator to work.
+// fb_addr is RBX from the loader, all sixty-four bits of it. It was declared
+// uint32, which cut an aperture above 4 GB down to whatever its low half
+// happened to be -- a plausible address below the hole, pointing at nothing
+// in particular -- and vga_fb_init() moved the console onto it. On a laptop
+// that is the screen stopping dead at the "vga:" line.
 int
-kmain(uint32 fb_addr)
+kmain(uint64 fb_addr)
 {
   // Carried out of the block below, for the framebuffer that could not be
   // attached there because it needs page tables that do not exist yet.