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.

references/fallback.md

@@ -0,0 +1,9 @@
+# Fallback
+
+`preflight.sh`: 0 full, 1 degraded, 2 no XeLaTeX/`article.cls`.
+
+- Exit 2: deliver `ledger.yaml` + Markdown; say there is no PDF.
+- No `pdftotext` / `pdftoppm`: only `--tex` / `--excerpt`.
+- No `jsonschema`: create `.venv` (`python3 -m venv .venv && .venv/bin/pip install -r requirements.txt`).
+- Sibling `build.sh` fails: drop that figure to `align` or screenshot, keep the notes compiling.
+- No `magick`: `screenshot.sh` must fail if `crop_bbox` is set; do not silently ship the full page.