scan: show each phase as N of M, with progress, elapsed time and ETA, also while it waits (closes #112)

Every scan display names its phase and its place among the five, as
"[2/5] walk", with its progress, elapsed time and ETA. load counts the
records first; the walk estimates its total from the records under the
operands; the content phase adds up the sizes of the matching groups
and says it is searching until the first record returns. Each display
redraws every 100 ms on a terminal and every 5 seconds in plain mode.

On a terminal no line reaches the last column: a line too wide drops
its rate, then its bar. After the terminal is made narrower than the
widest line drawn, the display makes a new library object, whose erase
fits.

Judgement call: load and the content checks draw the bar too, once their totals are known.
Judgement call: the bar is 10 columns wide, not the width the line leaves.

Model: opus-5-5

TODO.md

@@ -29,6 +29,21 @@
 
 # Completed Steps
 
+- each `scan` phase's progress display names the phase and its place among the
+  five, as `[2/5] walk`, from the moment it starts, with its progress, elapsed
+  time and ETA: `load` counts the records first, the walk's total is estimated
+  from the records under the operands, and the content phase counts the records
+  it checks from SQLite's grouping alone, then says it is searching the database
+  until the first one is returned. On a terminal a phase with an exact total
+  draws its line followed by the library's bar, and any other a spinner followed
+  by the line; every display is drawn again several times a second. No line
+  reaches the terminal's last column, however narrow, also after the terminal is
+  made narrower during a phase: a line too wide leaves out its rate, then its
+  bar, keeping the phase, progress, elapsed time and ETA whole, and is cut short
+  only when those alone do not fit, as below about 70 columns late in a large
+  scan. In plain mode the line is printed every 5 seconds whether or not an item
+  completed (2026-10-16, https://git.eeqj.de/sneak/sfdupes/issues/112)
+
 - `scan`, `report` and `trees` first print a line on stderr naming the database
   they use, as an absolute path, and whether that path is the default or comes
   from `SFDUPES_DATABASE`; `scan` says when the database is new. README