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/release.yml

@@ -35,7 +35,7 @@ jobs:
           sudo apt-get install -y --no-install-recommends \
             debootstrap squashfs-tools dosfstools \
             systemd-ukify systemd-boot kmod bubblewrap \
-            debian-archive-keyring python3-pip git zstd
+            debian-archive-keyring python3-pip git zstd cpio
           # Ubuntu's apt-packaged mkosi is too old to wire
           # non-free-firmware shorthand through to debootstrap.
           # mkosi isn't published on PyPI under v24+ — install the