Announce each operand on stderr before its passes

With per-operand walk/hash/update cycles, a multi-operand invocation
(e.g. scan /srv/*) showed pass totals that looked like the whole
run's: an operator watching operand 3 of 14 hash 300k files concluded
the other 20M files were being skipped. Print the operand path and
its position before each cycle.

TODO.md

@@ -19,6 +19,11 @@
 
 # Completed Steps
 
+- announce each operand on stderr before its passes (2026-07-24,
+  branch `scan-operand-progress`): with per-operand walk/hash/update
+  cycles, a multi-operand run (e.g. `scan /srv/*`) showed pass totals
+  that looked like the whole run's — an operator watching operand 3 of
+  14 hash 300k files concluded 20M files were being skipped
 - parallel walk (2026-07-24, branch `parallel-walk`): the walk pass
   was a single goroutine and took hours at ~20M files on a busy pool
   (observed: 22M files in 4h on a ZFS server); it is now a