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.

scripts/router.py

@@ -93,8 +93,8 @@ def check_row(
 
 def phase_from_env() -> int:
     import os
-    raw = os.environ.get("SUPERPAPER_PHASE", "1")
+    raw = os.environ.get("SUPERPAPER_PHASE", "2")
     try:
         return int(raw)
     except ValueError:
-        return 1
+        return 2