Parallelize the walk and commit per operand

Replace the single-goroutine WalkDir traversal with a per-directory
worker pool: workers read directories concurrently and lstat entries
while each directory is fresh in cache, recording size and mtime
during the walk. This folds the separate stat pass away (halving
metadata I/O per run) and overlaps metadata latency, which dominated
on busy pools — a sequential walk of a ~22M-file tree was observed
taking over 4 hours.

Each PATH operand now loads its scope, walks, hashes, and commits in
its own transaction, so an interrupted scan keeps every operand
completed so far; a later overlapping operand sees the records
committed by earlier ones and reuses them unchanged.

TODO.md

@@ -14,16 +14,19 @@
 
 # Next Step
 
-- 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
+- convert Makefile targets to scripts-to-rule-them-all `script/`
+  entrypoints like the other managed repos
 
 # Completed Steps
 
+- 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
+  per-directory worker-pool traversal that records size/mtime during
+  the walk (folding away the separate stat pass, halving metadata
+  I/O), and each `PATH` operand commits in its own transaction so an
+  interrupted scan keeps completed operands
+
 - persistent scan database (2026-07-24, branch `persistent-database`):
   `scan` now maintains a SQLite database (`modernc.org/sqlite`, pure
   Go, cgo stays disabled) keyed by absolute path that survives between
@@ -49,8 +52,6 @@
 
 # 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