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/20000000-0000-4000-8000-000000000002/50000000-0000-4000-8000-000000000005/index.md

@@ -0,0 +1,7 @@
+---
+schema: 1
+order: 1024
+title: Not Itself Deleted
+---
+My lane is tombstoned but I am not — isDeleted keys only on my own
+'deleted' field, never inherited from an ancestor.

Fixtures/Valid/tombstones.kanban/20000000-0000-4000-8000-000000000002/index.md

@@ -0,0 +1,10 @@
+---
+schema: 1
+order: 2048
+title: A Tombstoned Lane
+deleted: 2026-07-11T09:30:00Z
+---
+Everything beneath a tombstoned lane is hidden from the board too — but
+that's a rendering rule, not a loader rule: the loader still reports this
+lane's card structurally, un-flagged, since the card's own 'deleted' key
+is absent.