Attach the stepwise derivation toolkit as a submodule and default
SUPERPAPER_PHASE to 2 so rewrite-figure can use superderive. Notes
still keep align for short expansions without cancel or subst.
House the two figure toolkits as git submodules under SuperPaper.
The parent owns paper-note orchestration; children keep their own
history, macros, and lint. DESIGN.md is the family contract.