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.request.md

@@ -0,0 +1,14 @@
+# 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"}