[thekevinbot]

Fixes #1546
Refs #1461

A directory wider than one 8192-entry chunk was read whole, then sorted on one thread. Now, once a second chunk arrives, a thread of its own sorts each chunk while the next is read; the sorted chunks are then split at names sampled from the first chunk and merged on every core (a stable sort over each run's already-sorted shares). A directory of one chunk is sorted as before, with no thread. Only listing.rs changes; scanner.rs and tree_walk.rs are untouched.

Measurements

1.2M-entry flat tree (./docs and the root, every 50th file a dotfile), release builds, paired runs on a loaded host. Output md5 identical to main for ./docs, ./*, ./, ./docs/*, ./**/*.md, and a 74k-file deep tree.

PENDING

E2E Verification

PENDING

Changelog / Migrations

• Changelog fragment added under <root>/<pkg>/changelog.d/ for each changed package (or: skip-changelog trailer on a commit with reason)
• Migration fragment with the same slug added under <root>/<pkg>/migrations.d/ for each changed package (every subsection but Summary _None._ when nothing breaks)