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/index.md

@@ -0,0 +1,8 @@
+---
+schema: 1
+title: Coercion And Lenient-Field Fallback
+---
+Schema-owned display fields coerce where a sensible reading exists (a
+wrong-type scalar reads as its source text) and fall back to their default
+where none does (a sequence/mapping, a non-integer width). A present but
+unusable 'deleted' timestamp still tombstones — presence outranks validity.