The walk selection is built field by field from the caller's filters, so a
caller's limit, projection, order or cursor cannot reach it (one select per
member at limit 1 measured 300 ms against 14 ms). A test pins it.
Unverified at this commit: built and tested once at the end of the batch.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>