docs: beacon v1.1 spec — parser fidelity fixes, referee rework, equations, citations, notes

docs: beacon addendum — variable retrieval, text input, rename

docs: figures & captions spec addendum and plan (tasks 19-21)

docs(spec): record compact-corpus amendment from Task 11

docs: bibcode is the canonical lit identifier; arXiv ids optional

Adopt Pathfinder keyword-weighted retrieval per quick_pathfinder_query.py

Spec self-review fixes: hybrid score formula, embed score label

Add design handoff, prototype files, and approved design spec