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/10000000-0000-4000-8000-000000000001/20000000-0000-4000-8000-000000000002/index.md

@@ -0,0 +1,12 @@
+---
+schema: 1
+labels: [bug, urgent]
+order: 1024
+assignees: [bob, carol]
+title: Card With Unknowns
+due: 2026-08-15
+remote: {system: github, id: 42}
+agent-scratch: notes here
+---
+Card body describing the bug. Unknown/reserved keys are interleaved with
+schema-owned ones on purpose, to prove ordering survives the interleave.

Fixtures/Valid/unknown-key-order.kanban/10000000-0000-4000-8000-000000000001/index.md

@@ -0,0 +1,9 @@
+---
+schema: 1
+order: 1024
+remote-state: syncing
+title: Lane With Unknowns
+assignees: [alice]
+due: 2026-08-01
+custom: xyz
+---