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/figures/F3/F3-mechanism.md

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+$QK^\top \to QK^\top/\sqrt{d_k}$。缩放压方差，避免 softmax 饱和。

examples/derive-delegate/notes/figures/F3/F3.request.md

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+# Figure request F3
+toolkit: superderive
+language: cjk
+claim: 1/√d_k 是改写，不是新算子
+grammar: derivation
+work_rel_dir: figures/F3
+
+## Flow
+stage: {name: D1, text: "缩放来自方差，不是装饰"}
+row: {name: R1}
+in_row:
+  - {macro: sdstep, name: s1, role: rewrite,
+     lhs: "QK^{\\top}", rhs: "QK^{\\top}/\\sqrt{d_k}"}
+  - {macro: sdreason, name: s1, text: "§3.2.1，避免 softmax 饱和"}

examples/derive-delegate/notes/figures/F3/F3.tex

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+% Golden example -- a scale rewrite that must be seen, not just aligned.
+%   ../scripts/build.sh rewrite-cancel.tex
+\documentclass[border=10pt]{standalone}
+\usepackage[cjk]{superderive}
+
+\begin{document}
+\begin{tikzpicture}
+\sdstage{D1}{缩放来自方差，不是装饰}
+\sdrow{R1}
+  \sdstep[role=rewrite]{s1}{QK^{\top}}{QK^{\top}/\sqrt{d_k}}
+  \sdreason{s1}{§3.2.1，避免 softmax 饱和}
+\sdrowend
+\sdbbox{all}
+\sdtopformula{F}{$\mathrm{Attention}(Q,K,V)=\mathrm{softmax}(QK^{\top}/\sqrt{d_k})V$}
+\sdmeaningbox{mb}{120mm}{all}
+  {点积方差随 $d_k$ 线性涨}
+  {缩放是改写，不是新算子}
+  {没有缩放，softmax 进饱和区，梯度消失}
+\end{tikzpicture}
+\end{document}