Show the run log's last lines under Details, live

The percentage says an update is alive. It does not say what it is alive
doing, and for the longest stretch of a run there was no way to find out:
an AUR package compiling for a quarter of an hour talks constantly, and
every word of it went into a file nobody was looking at.

So the run log's last five lines now travel with the progress entry and
sit under Details, refreshed every two seconds for as long as the update
lasts. Polled rather than followed, for the same reason the pacman
watcher polls: only the last few lines are ever displayed, so every line
in between would be work nobody sees, and re-reading the tail whole makes
truncation and rotation of the log a non-event.

The job model behind this interface carries exactly two description
fields - descriptionValue1 and 2, there is no third - so one stays with
the package being worked on and the other takes the tail. Newlines inside
a field do render, which is what lets five lines share one of them. They
travel tab separated because the protocol is one instruction per line,
and a tab inside a log line becomes a space first: either renders as
whitespace, but a line split in half renders as nonsense. Five lines
clipped to 120 columns also keeps an instruction well inside PIPE_BUF,
which is what makes the write atomic against the runner sending its own
progress down the same pipe.

doc/cachy-auto-update.1.scd

@@ -131,6 +131,13 @@ stuck update. On a domestic line the download is then the longest of the three,
 and a bar that called the whole thing "installing" would sit near its beginning
 for minutes at a time looking stuck.
 
+Expanding "Details" shows the last five lines of the run log as they are
+written, alongside the package currently being worked on. The percentage says
+the update is alive; these lines say what it is alive doing, which matters most
+during the stretches that have nothing countable to report - an AUR package
+being compiled, above all. The job interface carries exactly two description
+fields, so those are the two things shown.
+
 A step that turns out to have no work is dropped from the bar instead of being
 handed its share for nothing. Packages already in the cache are never
 announced, so a run with everything already fetched skips the download step