live-image: fix check-initrd size measurement; add zstd to image

Previous run actually built the 518 MB rootfs with firmware-misc-nonfree
et al. installed — the real payload is working. Two follow-ups:

- check-initrd was reading stat on a symlink path and getting 30 bytes
  (the symlink's own size), not the 6.1.0-44-amd64 kernel initrd it
  points to. Switched to wc -c, which follows symlinks, and to du -hL
  for the OK message.
- Add zstd to Packages= so COMPRESS=zstd in initramfs.conf can be
  honored; without it update-initramfs falls back to gzip with a
  "No zstd in PATH" warning.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

live-image/mkosi.conf

@@ -45,6 +45,7 @@ Packages=
     pciutils
     usbutils
     initramfs-tools
+    zstd
     # Firmware. firmware-linux-nonfree on bookworm is a thin metapackage
     # that does NOT pull i915 GuC/HuC — those live in firmware-misc-nonfree.
     # Enumerate explicitly so the blob for whatever hardware we boot on