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.

examples/derive-delegate/notes/sections/sec-02.tex

@@ -0,0 +1,8 @@
+\section{主张与贡献}
+\splabel{C1}
+点积方差随 $d_k$ 涨。除掉 $\sqrt{d_k}$ 是改写，不是新算子。
+
+\spfig{F3}{缩放来自方差。}{fixture；toolkit: \texttt{superderive}；ledger id: F3。}
+
+\subsection{本章小结}
+这一步必须被看见，所以走 superderive，而不是笔记里的 \texttt{align}。