pn01, pn02: enable memtest86 and update stability docs

Enable memtest86 in systemd-boot menu on both PN51 units to allow
extended memory testing. Update stability document with March crash
data from pstore/Loki — crashes now traced to sched_ext scheduler
kernel oops, suggesting possible memory corruption.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

hosts/pn01/configuration.nix

@@ -12,6 +12,7 @@
   ];
 
   boot.loader.systemd-boot.enable = true;
+  boot.loader.systemd-boot.memtest86.enable = true;
   boot.loader.efi.canTouchEfiVariables = true;
 
   networking.hostName = "pn01";