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/unknown-key-order.kanban/index.md

@@ -0,0 +1,12 @@
+---
+schema: 1
+project: lanework
+title: Unknown Key Ordering
+sphere: work
+template: {order: 5}
+labels: [x, y, z]
+custom-note: keep me
+---
+Unknown keys and their relative order are preserved verbatim, on every
+level — this is load-bearing for agent overlays (project:, sphere:,
+custom tags).