live-image: pack full rootfs as initrd so PXE actually boots userspace

update-initramfs produces a boot stub (~50 MB) that expects to mount a
separate rootfs over squashfs/disk/NFS. Our PXE channel only ships
vmlinuz+initrd.img, so the stub had nothing to pivot to — kernel
finished hand-off and the system wedged with firmware, modules, and
userspace stranded in the 545 MB rootfs dir we never delivered.

Replace with an everything-in-initramfs build: cpio.zst the full
rootfs (minus /boot) as the initrd, add /init -> sbin/init for the
kernel's runtime entrypoint, materialize the kernel symlink into a
real file. Bump check-initrd floor to 200 MB and switch the firmware
grep from unmkinitramfs (boot-stub-specific) to zstd | cpio -t.

Also add cpio to the CI apt deps.

.gitea/workflows/e2e.yml

@@ -39,7 +39,7 @@ jobs:
             debootstrap squashfs-tools \
             systemd-ukify systemd-boot kmod bubblewrap \
             debian-archive-keyring python3-pip git zstd \
-            qemu-system-x86 qemu-utils \
+            qemu-system-x86 qemu-utils cpio \
             dnsmasq iperf3 ipxe-qemu
           # See release.yml for rationale — Ubuntu's apt mkosi is too old
           # to handle bookworm's non-free-firmware component correctly.