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/tombstones.kanban/index.md

@@ -0,0 +1,7 @@
+---
+schema: 1
+title: Tombstoned Lane And Card Still On Disk
+---
+Deletion writes 'deleted:' into frontmatter; the item never leaves disk.
+The loader keeps tombstoned items in the snapshot, flagged — hiding them
+from the board is a rendering concern, not a loading one.