Correct four inaccurate comments in cancel_test.go and rename a constant (closes #33)

Fix the comments the re-review of
#6 found misdescribing their
tests; no test's behaviour changes.

State the property the walkClock tests rely on, that the index load's
Done cost does not grow with the record count, instead of a wrong fixed
figure. Describe poolUnwind so it is true of every use, and mark the
tests that have no bound. Record that hashWorker's results-send exit is
reachable through stop but has no test that fails without it. Rename
walkCancelInFlightDirs to walkCancelInFlightFiles, since it counts
files. Note at the top why the file departs from the
one-test-file-per-source-file convention.

Model: opus-4-8 (implementation); opus-5-5 (rework)

TODO.md

@@ -29,6 +29,10 @@
 
 # Completed Steps
 
+- correct four inaccurate comments in `cancel_test.go` and rename
+  `walkCancelInFlightDirs` to `walkCancelInFlightFiles` (2026-10-04,
+  https://git.eeqj.de/sneak/sfdupes/issues/33)
+
 - test the `-x` filesystem-boundary rules in `subdirJob` (2026-10-04,
   https://git.eeqj.de/sneak/sfdupes/issues/17)