refactor(quandledb): extract to top-level monorepo

quandledb has been promoted to a top-level monorepo at
github.com:hyperpolymath/quandledb.git. This monorepo's role narrows
to host the other nextgen-databases projects; quandledb stands on its
own from 2026-04-19.

Per-file history for quandledb/ remains in this monorepo's git log
up to the extraction commit (this one) for bisect and blame.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath