Specify parallel per-directory walk and per-operand commits

The walk pass is a single goroutine; on a busy ZFS pool it manages
only a few thousand directory entries per second and takes hours at
~20M files. Respecify it as a worker-pool traversal that reads
directories concurrently and records size/mtime during the walk,
folding away the separate stat pass and halving metadata I/O. Each
PATH operand now commits in its own transaction so an interrupted
scan keeps the operands completed so far.

TODO.md

@@ -14,8 +14,13 @@
 
 # Next Step
 
-- convert Makefile targets to scripts-to-rule-them-all `script/`
-  entrypoints like the other managed repos
+- parallel walk (branch `parallel-walk`): the walk pass is a single
+  goroutine and takes hours at ~20M files on a busy pool (observed:
+  22M files in 4h on a ZFS server); make it a per-directory
+  worker-pool traversal that records size/mtime during the walk
+  (folding away the separate stat pass, halving metadata I/O), and
+  commit each `PATH` operand in its own transaction so an interrupted
+  scan keeps completed operands
 
 # Completed Steps
 
@@ -44,6 +49,8 @@
 
 # Future Steps
 
+- convert Makefile targets to scripts-to-rule-them-all `script/`
+  entrypoints like the other managed repos
 - possible later features (explicitly out of scope per README):
   full-content verification of candidates, removal-script helpers