Extract biometric-puzzle as canonical reference repo (post-UBMK) — thesis signature artifact

Move the challenge library + client scoring (TS) + server re-scoring (Py) into this repo
as the canonical reference impl, with thin adapters back into web-app/biometric-processor.
Follow multi-repo restructure rules (destination PR first, source PR last, line-count check).
Pairs with UBMK paper + CITATION.cff + JOSS. Post-UBMK timing.
From POST_THESIS_BACKLOG.md "Architecture decision".