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

Every scan display now names its phase and its place among the five,
as "[2/5] walk", with its progress, rate, 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 is drawn from its own goroutine: several times a
second on a terminal, every 5 seconds in plain mode.

On a terminal a phase with an exact total draws its line before the
library's bar when there is room, and any other a spinner before the
line. A line too wide drops its rate, then its bar; it is cut only when
the phase, progress, elapsed time and ETA alone do not fit.

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: 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