feat: land superpaper v1 notes scaffold

Add the ledger schema, class router, lint codes, ingest/build
pipeline, and three work-tree examples: align derivation,
superfig delegation, and supertensor delegation.

examples/excerpt-toy/ledger.yaml

@@ -0,0 +1,46 @@
+schema: superpaper.ledger/v1
+retired_ids: []
+paper:
+  id: excerpt-toy
+  title: A One-Step Predictor
+  authors: ["Fixture"]
+  year: 2026
+  venue: Superpaper examples
+  notes_language: zh
+  source: {kind: excerpt}
+coverage:
+  mode: excerpt
+  sections_in: ["1"]
+questions:
+  - {id: Q1, text: "一次前向如何得到预测，损失该放在哪？", source: "excerpt"}
+claims:
+  - id: C1
+    text: "一次前向是 x → f_θ → ŷ；损失在预测之后单独计算"
+    kind: contribution
+    status: core
+    supports: [Q1]
+    source: "excerpt"
+definitions:
+  - {id: D1, name: "one-step predictor", text: "ŷ = f_θ(x)", source: "excerpt"}
+assumptions: []
+lemmas: []
+symbols:
+  - {name: x, latex: "x", meaning: "输入", kind: value, introduced: "excerpt"}
+  - {name: yhat, latex: "\\hat y", meaning: "预测", kind: value}
+  - {name: L, latex: "L", meaning: "损失", kind: scalar}
+  - {name: d, latex: "d", meaning: "特征维", kind: "shape parameter"}
+derivations:
+  - id: DER1
+    claim: C1
+    title: "缩放来自方差"
+    source: "excerpt"
+    expand: true
+    figure: null
+    steps:
+      - {id: S1, from: "u^\\top v", to: "u^\\top v / \\sqrt{d}", rule: scale,
+         justify: "点积方差随 d 增长"}
+figures: []
+evidence: []
+terms:
+  - {canonical: "one-step predictor", aliases: ["一次前向"]}
+source_assets: []