Show the progress rate over the last 5 seconds, and what a phase is doing 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, and falls to 0
while nothing completes; the ETA still comes from the average. Once no
item has completed for 10 seconds, the line says so and what the phase
is doing, a phrase each phase gives where it is defined. A narrow
terminal leaves that out after the rate, keeping the phase, progress,
elapsed time and ETA whole. The tests set the display's clock by hand.

Judgement call: the line reads "no files finished", in the phase's unit.
Judgement call: trees' group phase says "grouping directories".

Model: opus-5-5

README.md

@@ -833,17 +833,33 @@ Each display is one line holding:
 - a `[m/n] x%` display (items processed / total items, percent); before a phase
   has a total, the items processed, or what the phase is waiting for
 - current rate (items/s), naming the items the phase counts: files, records,
-  directories or groups
+  directories or groups. It covers the items completed over the last 5 seconds,
+  or, when stderr is not a terminal, since the line before, so it falls to 0
+  while nothing completes, and a burst of items after a wait shows as a burst
+  rather than as a steady rate.
+- once no item has completed for 10 seconds, how long that has been and what the
+  phase is doing (see below)
 - elapsed time
-- estimated time remaining, or `unknown`
+- estimated time remaining, or `unknown`. It comes from the phase's average rate
+  since it started counting, so that it does not swing with each burst.
 
 Example shape (exact layout is flexible, content is not):
 
 ```
 [3/5] hash: [12345/98765] 12% |████      | 92 files/s elapsed 2:32 eta 17:54
 [2/5] walk: [1234567/~1200000] 103%, 5120 files/s, elapsed 4m1s, eta unknown
+[3/5] hash: [12345/98765] 12%, 0 files/s, no files finished for 42s, reading files, elapsed 4m1s, eta 28m7s
 ```
 
+When no item has completed for 10 seconds, as while one large file is read, a
+disk is slow or the database is busy, the line says so and what the phase is
+doing: `walk` is listing directories; `hash` and the content phase's reads are
+reading files; `update` is writing to the database; `load` and the `read` phases
+of `trees` and `report` are reading the database; the content phase's checks are
+searching the database; `trees`' `group` is grouping directories, and its
+`write` is writing the report. A phase still waiting for its total already says
+what it waits for, and says nothing more.
+
 Additional requirements:
 
 - On a terminal, a phase with an exact total (every phase but `scan`'s `walk`,
@@ -857,13 +873,14 @@ Additional requirements:
 - On a terminal no line reaches the terminal's last column, however narrow the
   terminal is. The phase and its place, the progress, the elapsed time and the
   ETA are kept whole: a line too wide for the terminal leaves out its rate first
-  (or what the phase is waiting for), and a bar with no room beside those four
-  gives way to the spinner. Only when those four alone are too wide, as on a
-  terminal narrower than about 70 columns hours into a scan of millions of
-  files, is the line cut short at the terminal's edge, so the end of the ETA is
-  lost first. A terminal made narrower while a phase runs gets the same: the
-  next draw erases the line within the new width and fits the line to it, and
-  the display goes on redrawing in place.
+  (or what the phase is waiting for), then what it says of 10 seconds with no
+  item completed, and a bar with no room beside those four gives way to the
+  spinner. Only when those four alone are too wide, as on a terminal narrower
+  than about 70 columns hours into a scan of millions of files, is the line cut
+  short at the terminal's edge, so the end of the ETA is lost first. A terminal
+  made narrower while a phase runs gets the same: the next draw erases the line
+  within the new width and fits the line to it, and the display goes on
+  redrawing in place.
 - When stderr is not a terminal (a pipe, a file, `/dev/null`), do not emit ANSI
   redraws: print the line the moment each phase starts, every 5 seconds while it
   runs (whether or not an item has completed, and never more often), and once