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/ledger.yaml

@@ -0,0 +1,46 @@
+schema: superpaper.ledger/v1
+retired_ids: []
+paper:
+  id: derive-delegate
+  title: Scaled attention
+  authors: ["Fixture"]
+  notes_language: zh
+  source: {kind: excerpt}
+coverage:
+  mode: excerpt
+questions:
+  - {id: Q1, text: "缩放从哪来？", source: "excerpt"}
+claims:
+  - id: C1
+    text: "1/√d_k 是改写，不是新算子"
+    kind: theoretical
+    status: core
+    supports: [Q1]
+definitions: []
+assumptions: []
+lemmas: []
+symbols:
+  - {name: Q, latex: "Q", meaning: "query", kind: value}
+  - {name: K, latex: "K", meaning: "key", kind: value}
+  - {name: dk, latex: "d_k", meaning: "key 维", kind: "shape parameter"}
+derivations:
+  - id: DER1
+    claim: C1
+    title: "缩放"
+    expand: true
+    figure: F3
+    steps:
+      - {id: S1, from: "QK^{\\top}", to: "QK^{\\top}/\\sqrt{d_k}", rule: scale}
+figures:
+  - id: F3
+    claim: C1
+    title: "缩放来自方差"
+    grammar: derivation
+    toolkit: superderive
+    signals: [rewrite-figure]
+    request: figures/F3/F3.request.md
+    include: figures/F3/build/F3.pdf
+    status: included
+evidence: []
+terms: []
+source_assets: []