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

Every scan display now starts with its phase and its place among the
five, as "[2/5] walk", and shows its progress, rate, elapsed time and
ETA. On a terminal hash, update and the content reads keep the
library's bar, which fills what the rest of the line leaves; load,
the walk and the content checks are a spinner whose text is cut to
the terminal's width. load counts the records first. The content
phase adds up the sizes of the matching groups, without the join and
sort, and says it is searching until the first record returns. The
walk's total is estimated from the records under the operands. Each
display is drawn from its own goroutine: several times a second on a
terminal, every 5 seconds in plain mode.

Judgement call: load and the content checks stay spinners once their totals are known.

Model: opus-5-5

TODO.md

@@ -29,6 +29,18 @@
 
 # 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 `hash`, `update` and the
+  content phase's reads keep the library's bar and the other displays are a
+  spinner followed by the line; every display is drawn again several times a
+  second, and every line fits the terminal's width. 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