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/coercion.kanban/10000000-0000-4000-8000-000000000001/30000000-0000-4000-8000-000000000003/index.md

@@ -0,0 +1,6 @@
+---
+schema: 1
+order: 1024
+title: 2048
+---
+An unquoted integer title coerces to its source text, "2048".

Fixtures/Valid/coercion.kanban/10000000-0000-4000-8000-000000000001/40000000-0000-4000-8000-000000000004/index.md

@@ -0,0 +1,7 @@
+---
+schema: 1
+order: 2048
+title: [a, b]
+---
+A sequence has no sensible string reading — malformed, falls back to the
+untitled placeholder.

Fixtures/Valid/coercion.kanban/10000000-0000-4000-8000-000000000001/50000000-0000-4000-8000-000000000005/index.md

@@ -0,0 +1,7 @@
+---
+schema: 1
+order: 3072
+iconColor: 42
+---
+An integer iconColor coerces to its source text, "42" — not a real color,
+but the value round-trips rather than being rejected.

Fixtures/Valid/coercion.kanban/10000000-0000-4000-8000-000000000001/60000000-0000-4000-8000-000000000006/index.md

@@ -0,0 +1,7 @@
+---
+schema: 1
+order: 4096
+background: {x: 1}
+---
+A mapping has no sensible string reading — malformed, falls back to no
+color.

Fixtures/Valid/coercion.kanban/10000000-0000-4000-8000-000000000001/70000000-0000-4000-8000-000000000007/index.md

@@ -0,0 +1,5 @@
+---
+schema: 1
+order: 5120
+background: 12345
+---

Fixtures/Valid/coercion.kanban/10000000-0000-4000-8000-000000000001/80000000-0000-4000-8000-000000000008/index.md

@@ -0,0 +1,8 @@
+---
+schema: 1
+order: 6144
+title: Deleted With An Unusable Timestamp
+deleted: definitely-not-a-date
+---
+The user's intent to delete outranks the broken date — this card still
+tombstones, its deletion date just unknown.

Fixtures/Valid/coercion.kanban/10000000-0000-4000-8000-000000000001/index.md

@@ -0,0 +1,6 @@
+---
+schema: 1
+order: 1024
+title: Width Coerces From A Quoted Number
+width: "3"
+---