docs(lancedb): mark the husk sweep removable once lance#8322 ships

[gloryfromca]

Follow-up to #392. Upstream acted on one of the two issues that PR's investigation produced.

lance-format/lance#8320 — cleanup_old_versions unlinks a superseded index's files but never removes the directory — was fixed by lance#8322, merged 2026-08-06. It removes the emptied directories under the same 7-day / delete_unverified policy that _remove_empty_index_dirs reimplements on our side.

The fix is not in any release yet: lancedb 0.34.0 embeds lance 8.0.0, and the merge commit sits ahead of v11.0.0-beta.2. So the sweep stays for now. This records why it should not stay forever, next to the code rather than in a chat log.

Two consequences worth having written down at the point of use:

• Delete, don't keep both. Once a pin carries #8322, _remove_empty_index_dirs and its call site should go — a second implementation of upstream's own rule is the kind of thing that survives for years because nobody remembers it was a workaround.
• The documented horizon halves. The constant currently explains a ~14-day steady state, because lance frees the files at 7 days, which resets the directory's mtime, and only then does our 7-day gate start. Upstream drops the directory in the same pass that empties it, so the two windows stop stacking and it goes back to ~7 days (~890k dirs / ~3.6GB at ceiling load, from ~1.8M / ~7GB).

Docs only — no behaviour change. ruff clean, 1938 unit tests passing.

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