nixos: add Memtest86+ GRUB boot entry

Enable boot.loader.grub.memtest86 so a Memtest86+ entry appears in the
GRUB menu, for diagnosing suspected DRAM bit-flips on this non-ECC box
(recurring RTS heap-corruption SIGSEGVs whose fault pattern looks like a
single-bit flip in an otherwise-valid pointer).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

nixos/bootloaders/grub.nix

@@ -62,6 +62,7 @@ in {
 
       grub = {
         enable = true;
+        memtest86.enable = true;
         efiSupport = true;
         device = "nodev";
         useOSProber = cfg.windowsEfiUuid == null;