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

@@ -0,0 +1,6 @@
+---
+schema: 1
+order: 1024
+title: A Card With Junk Inside It
+---
+Body.

Fixtures/Valid/stray-files.kanban/10000000-0000-4000-8000-000000000001/20000000-0000-4000-8000-000000000002/notes/whatever.txt

@@ -0,0 +1 @@
+this whole subfolder is never scanned — cards are leaves

Fixtures/Valid/stray-files.kanban/10000000-0000-4000-8000-000000000001/20000000-0000-4000-8000-000000000002/scratch.md

@@ -0,0 +1 @@
+scratch

Fixtures/Valid/stray-files.kanban/10000000-0000-4000-8000-000000000001/draft.txt

@@ -0,0 +1 @@
+draft notes

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

@@ -0,0 +1,5 @@
+---
+schema: 1
+order: 1024
+title: A Lane With Junk Beside It
+---

Fixtures/Valid/stray-files.kanban/README.txt

@@ -0,0 +1 @@
+board readme

Fixtures/Valid/stray-files.kanban/index.md

@@ -0,0 +1,7 @@
+---
+schema: 1
+title: Stray Files Everywhere
+---
+Stray files at board, lane, and card level are ignored — preserved
+verbatim, never rendered, never warned about (only directories are level
+candidates at all).

Fixtures/Valid/stray-files.kanban/notes.log

@@ -0,0 +1 @@
+log line