v0.7.0: save files and settings on the USB stick (Ubuntu)

Persistence: files and settings saved back to the USB stick (Ubuntu).

• persistence: true: the boot menu's first entry keeps the live system's changes on the stick. That covers files, settings and apps added. casper makes a partition labelled writable in the free space after the ISO on the first boot.
• A second entry starts clean and leaves what was saved alone. Install always installs the clean system.
• CI test: the ISO is copied onto a bigger stick image and booted twice (boottest modes persist-1 and persist-2). The second boot must find what the first wrote, both the system's file and the live user's. verify-results has a new persistence check, so a passing run now prints VERIFY OK iso,boot,session,install,persistence.
• Not on Fedora yet. Fedora-based specs are refused, with the reason. dmsquash-live keeps a persistent overlay only on a partition made beforehand, and nothing makes one on a written ISO. Doing it would mean repartitioning the boot stick from the initramfs.

CI: run 38936347364 (all nine examples; settings-ubuntu boots a writable stick twice and finds the first boot's files).