feat: add superderive child and open phase-2 routing

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.

references/figure-request.md

@@ -4,6 +4,8 @@ Sibling figure agents see only `notes/figures/F*/F*.request.md` plus that siblin
 
 - superfig: `\sfnode` keys `role, level, gap, bracket, at=`. `\sfconn` has no endpoints. `at` must be TikZ calc with `($…$)`.
 - supertensor: every `ststack` / `stface` / `stglyph` has `coord: ""`.
+- superderive: `\sdstep` / `\sdreason` / `\sdcancel` / `\sdsubst`; one `\sdcol` max.
+
 - `allow-raw-tikz` must also appear as `% superfig-lint: allow-raw-tikz` in the `.tex`.
 - Build: `superfig/scripts/build.sh F1.tex F1/build` (second arg is the outdir).
 - Screenshots: `scripts/screenshot.sh --work <work> --id F2` (`pdftoppm -r 200`, then optional `crop_bbox`).