list-profiles: show when each profile was last running

[Yury-MonZon]

list-profiles said when a profile was created but not whether it had ever been used, which is the
question you actually have with five bootable roots on one device.

Nothing records that directly, and @var-log is shared, so the journal cannot tell profiles apart.
What is per-profile is systemd's own state: var/lib/systemd/random-seed is rewritten at every boot
and again at clean shutdown, and var/lib/systemd/timesync/clock is rewritten by timesyncd while the
profile runs. The newest of the two is when that profile was last alive.

Existence alone is not the test. A profile inherits both files from whatever it was made from, so a
clone of a used profile, or an image whose build happened to create them, would otherwise report the
ancestor's time as if this profile had run. Only a stamp newer than the profile's own creation counts;
anything older reads never.

Accuracy, stated plainly rather than implied: exact for a cleanly shut down profile, within
timesyncd's poll interval for one that was cut off, and never worse than its boot time. It says the
profile was RUNNING, not that a person used it, which is what the header comment says so it cannot be
read as a login time.

NAME                      KIND     ID   CREATED              LAST USED            RO  PARENT
@Desktop                  profile  265  2026-07-29 10:08:34  2026-07-31 12:43:35  rw  @Desktop_777_stock (264)
@MigDst                   profile  305  2026-07-31 12:57:48  never                rw  @Desktop_778_stock (300)
@Minimal                  profile  263  2026-07-29 10:06:10  2026-07-31 12:45:40  rw  @Minimal_777_stock (262)
@Router                   profile  269  2026-07-29 10:08:55  never                rw  @Router_777_stock (268)

Verified on 192.168.1.110 across three profiles that were actually booted (Desktop, Minimal, then the
migrated profile) and from a third OS, the SD-card Installer, reading the unmounted eMMC through -d:

• the booted profile tracks the current time, profiles never booted read never
• a stamp planted before a profile existed is ignored; one planted after it is shown
• a profile replaced from its _stock reads never again, while the copy moved aside keeps its time
• the existing 73-check suite still passes, so no assertion shifted from the added column

Draft because it touches list-profiles, which #144 and #145 also change. If those land first this
needs a trivial rebase; if this lands first they get the same one-line conflict on the header printf.