First implementation (#1)

Reviewed-on: #1

fixtures/src/README.md

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+# Fixture sources
+
+Each source describes a document as a converter first read it, mistakes included, in a shorthand that `tools/build.py` expands.
+The build mints IDs, applies the listed corrections, lays out synthetic geometry, signs the listed receipts and writes the package.
+The shorthand is tooling, not a format.
+
+| Shorthand | Becomes |
+| --- | --- |
+| `{"p": [...]}` | A paragraph of the listed sentences. |
+| `{"s": [...]}` | A sentence; a string part is a text run. |
+| `{"sup": t}`, `{"sub": t}`, `{"em": t}` | A styled text run. |
+| `{"m": src}`, `{"t": src}` | A TeX or Typst math run. |
+| `{"cite": [...], "text": t}`, `{"xref": target, "text": t}`, `{"fn": name, "text": t}` | Citation, cross-reference and footnote-mark runs. |
+| `{"eq": {...}}` | A display equation inside the sentence. |
+| `{"row": [...]}`, `{"c": [...]}` | A table row, and a cell with `role`, `rowspan`, `colspan` and `col`. |
+| `"name": "@x"` | A local name, replaced by the minted ID; `ext:<doc>:@x` names a node of another source. |
+| `"page": n` | The page the node and its descendants are laid out on. |
+
+`corrections` lists the operations in order, naming nodes by local name; `crop` names the node whose crop is the evidence.
+`anchors_at_rev0` lists the anchors whose resolution `vectors/resolve.jsonl` records, and `receipts` the receipts, spot-checks and revocations to sign.