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/duplicate-order-tie-break.kanban/10000000-0000-4000-8000-000000000001/10000000-0000-4000-8000-000000000001/index.md

@@ -0,0 +1,5 @@
+---
+schema: 1
+order: 1024
+title: Should Render First (folder starts with 1)
+---

Fixtures/Valid/duplicate-order-tie-break.kanban/10000000-0000-4000-8000-000000000001/20000000-0000-4000-8000-000000000002/index.md

@@ -0,0 +1,5 @@
+---
+schema: 1
+order: 1024
+title: Should Render Second (folder starts with 2)
+---

Fixtures/Valid/duplicate-order-tie-break.kanban/10000000-0000-4000-8000-000000000001/30000000-0000-4000-8000-000000000003/index.md

@@ -0,0 +1,5 @@
+---
+schema: 1
+order: 1024
+title: Should Render Third (folder starts with 3)
+---

Fixtures/Valid/duplicate-order-tie-break.kanban/10000000-0000-4000-8000-000000000001/index.md

@@ -0,0 +1,5 @@
+---
+schema: 1
+order: 1024
+title: Lane With Three Tied Cards
+---

Fixtures/Valid/duplicate-order-tie-break.kanban/aaaaaaaa-aaaa-4aaa-8aaa-aaaaaaaaaaaa/index.md

@@ -0,0 +1,5 @@
+---
+schema: 1
+order: 2048
+title: Should Render Before Lane bbb... (same order, 'a' < 'b')
+---

Fixtures/Valid/duplicate-order-tie-break.kanban/bbbbbbbb-bbbb-4bbb-8bbb-bbbbbbbbbbbb/index.md

@@ -0,0 +1,5 @@
+---
+schema: 1
+order: 2048
+title: Should Render After Lane aaa... (same order, 'b' > 'a')
+---

Fixtures/Valid/duplicate-order-tie-break.kanban/index.md

@@ -0,0 +1,7 @@
+---
+schema: 1
+title: Duplicate Order Values, Tie-Broken By Folder Name
+---
+Two siblings that share an 'order' render in a deterministic order —
+ascending 'order' first, folder name (lexicographic) as the tiebreak.
+This board exercises the tiebreak at both lane level and card level.