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

Fixtures/Valid/rich-board.kanban/index.md

@@ -0,0 +1,22 @@
+---
+schema: 1
+# board settings, hand-written
+title: "Rich Demo Board"
+created: 2026-07-01T09:00:00Z
+modified: 2026-07-26T16:41:38Z
+modified-by: claude
+background: "#1E1E1E"
+icon: rectangle.stack.fill
+iconColor: purple
+
+project: lanework       # agent overlay
+sphere: work
+labels: [feature, docs]
+template: {order: 3}
+# trailing note before close
+---
+A demo board exercising the full breadth of the storage contract: several
+lanes, several cards, body text, styling, and unknown/reserved frontmatter
+keys living alongside the schema-owned ones.
+
+Nothing here is special-cased — it's an ordinary board.