many thanks for the suggestions.
tried sudo mkinitcpio -g /boot/initramfs-linux-zen.img and then sudo update-grub while chrooted from live usb. unfortunately the same issue continued to occur.
booted again to live usb, chrooted and ran sudo pacman -Syy linux-lts linux-lts headers . system successfully boots with linux-lts kernel. thank you.
will follow up with Issues · zen-kernel/zen-kernel · GitHub unless there is another recommended follow up. for the record the kernel on the live usb is 5.14.14-zen1-1, which i presume is what is installed and functioned correctly on first boot? the mount: /new_root can't find UUID issue began after upgrading to 5.15.2.zen1-1.