Write the loader fixture suite, valid and malformed

18 golden fixture boards as real on-disk trees under Fixtures/ — ten
valid/tolerated cases (rich board, interrupted create, non-UUID strays,
stray files, tombstones, tie-breaks, unknown-key order, coercion,
last-wins duplicates, board-level deleted) and eight fail-fast cases,
each pinned to its exact error reason and path; the rich board's whole
tree round-trips byte-identically. Pins two contract subtleties now
recorded in the design: tombstone flags are not inherited (renderers
walk ancestors) and last-wins resolves before order strictness. README
gains the storage-foundation feature entry. 146 tests total.

Claude-Session: https://claude.ai/code/session_018BjQRYBR6jQja3jCRi5S3A

README.md

@@ -8,7 +8,9 @@ The defining consequence: anything that can read and write files is a first-clas
 
 Lanework is in early development. This list tracks what has actually shipped and grows milestone by milestone; the full design lives in [DESIGN/](DESIGN/).
 
-*Nothing user-facing has landed yet — the project is at the scaffold stage.*
+*No UI yet — the storage foundation is in place:*
+
+- **Storage contract, read side** — frontmatter engine with a byte-perfect round-trip guarantee (unknown keys, comments, and formatting survive every rewrite; duplicate keys read last-wins; wrong-type scalars coerce read-side), gapped fractional ordering (Ranks), and a fail-fast board loader with UUID-gated level detection, warning-collecting skips, and tombstone-aware snapshots — pinned by a golden fixture suite of 18 on-disk boards.
 
 ## Development