Show the progress rate over the last 5 seconds, and where a phase waits after 10 seconds with no item done (closes #125)

The rate was the phase's average, so a burst after a long wait read as
over 1,000 files a second. It now covers the items completed over the
last 5 seconds, or since the line before off a terminal; the ETA still
comes from the average. Once no item has completed for 10 seconds, the
rate gives way to "stalled" and where the phase waits, as "stalled
disk/db": disk, db, out (stdout) or cpu, or two of them. A line too
wide for the terminal loses its bar first, then its rate or those
words, so the phase, progress, elapsed time and ETA stay whole. The
content phase's last partial batch is now written under a display of
its own. The tests set the display's clock by hand.

Deviation: the stall words hold no time and name where the phase waits, not what it does.
Judgement call: trees' group phase, which works in memory, says "cpu".

Model: opus-5-5

TODO.md

@@ -29,6 +29,22 @@
 
 # Completed Steps
 
+- the rate a progress display shows covers the items completed over the last 5
+  seconds, or since the line before when stderr is not a terminal, so it falls
+  to 0 while nothing completes, and a burst after a wait no longer reads as a
+  steady rate of over 1,000 files a second; the ETA still comes from the phase's
+  average. Once no item has completed for 10 seconds, the word `stalled` and
+  where the phase waits take the rate's place, as `stalled disk/db`: `disk`,
+  `db`, `out` for stdout or `cpu`, or two of them for a phase that may wait on
+  either, each phase giving its word where it is defined. A line too wide for
+  the terminal loses its bar first, then its rate or those words, so the phase,
+  progress, elapsed time and ETA stay whole. On an 80-column terminal, at counts
+  like those of README's example, the content phase's checks and reads lose
+  those words at most times in minutes, and every other phase keeps them
+  whenever the elapsed time and the ETA are both in minutes. The content phase's
+  last partial batch is now written under a display of its own (2026-10-16,
+  https://git.eeqj.de/sneak/sfdupes/issues/125)
+
 - new subcommand `info` prints the database's details on stdout, one per line as
   a name, a tab and the value: its path and where it came from, the sizes of the
   database file and of its `-wal` file when there is one, when the database file