Show progress while loading the record index

On a database with tens of millions of records, indexing the existing
rows before the walk takes real single-core time with no output,
which is indistinguishable from a hang. Give the load its own
spinner, and render every phase display the moment the phase starts
instead of waiting for its first completed item.

README.md

@@ -395,9 +395,11 @@ all dupe rows) in human units.
 Use the progress-bar library for all scan progress; rendering in the
 style of `pv` is the model. All progress goes to stderr.
 
-Each phase gets its own display. The walk has no known total while
-running: show a live file count, rate, and elapsed time
-(spinner-style, no percentage or ETA). The hash and update phases
+Each phase gets its own display, rendered the moment the phase
+starts — a scan must never look hung. Loading the existing-record
+index (`load`) and the walk have no known totals while running: show
+a live count, rate, and elapsed time (spinner-style, no percentage or
+ETA). The hash and update phases
 have exact totals — only files that actually need hashing appear in
 the hash total, so its ETA is meaningful. Required elements for the
 bars with known totals: