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.

.gitmodules

@@ -4,3 +4,7 @@
 [submodule "supertensor"]
 	path = supertensor
 	url = ssh://git@ssh.gitea.mygoband.com:2222/carrydela/SuperTensor.git
+
+[submodule "superderive"]
+	path = superderive
+	url = ssh://git@ssh.gitea.mygoband.com:2222/carrydela/SuperDerive.git