feat: vmtest.sh: the boot loader test for every loader the ISO advertises

One VM per loader, Steam Machine (--fremont): Steamify's state as its menu sees it,
OS name untouched, loader identity, the DKMS modules for every kernel, Steamify's
loader-specific code (hdmi_remove_boot_param), a kernel update, and a reboot into
each of the two kernels. Headless by default with a log Konsole (--window shows
the VMs). Fails when an advertised loader has no test. Limine 41, systemd-boot 40,
GRUB 40 checks pass.

.gitignore

@@ -7,6 +7,8 @@
 !/run.sh
 !/get-iso.sh
 !/TESTPLAN.md
+!/TODO.md
+!/.github/
 !/scripts/
 !/share/
 !/.claude/
@@ -15,6 +17,7 @@ share/*
 !share/guest-ssh-setup.sh
 !share/vminstall-live.sh
 !share/vminstall-post.sh
+!share/bootloader-test/
 
 # Belt and braces, anywhere in the tree
 *.iso