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/notes.tex

@@ -0,0 +1,14 @@
+\documentclass[a4paper]{article}
+\input{notes-macros}
+\renewcommand{\notetitle}{缩放来自方差}
+\renewcommand{\notepaper}{Scaled attention}
+\begin{document}
+\tableofcontents
+\newpage
+\input{sections/sec-01.tex}
+\input{sections/sec-02.tex}
+\input{sections/sec-03.tex}
+\appendix
+\section{符号表}
+\input{sections/symbols.tex}
+\end{document}