Relocate loose card files into attachments

01's Lanework-owns-the-board carve-out: a regular file beside a card's
index.md belongs in attachments/, and the app moves it there. The
loader detects read-only — a new LoadResult.looseCardFiles channel,
separate from the stray-tolerance warnings because it says the opposite
thing — skipping directories, symlinks, hidden entries, and the
reserved names compared case-insensitively (on APFS, Index.md IS the
index). The relocation rides one performWrite bracket at the tail of
every successful reload, which makes lock deferral free: the reload
that lifts a read-only lock is the reload that relocates. A
lane/card/filename memo keeps a failing relocation from hot-looping —
one one-shot, then silence until disk changes. The notice rides the
loss-row class, phrasing folded by BannerCenter (one file, one card's
files, a multi-card sweep), naming original filenames per the
importAttachment rule. Paste normalizes at the import boundary: staged
snapshots' loose files land in the pasted card's attachments silently,
every arrival path declaring its side via an explicit
normalizingLooseFiles parameter — drag paths decline and fall back to
the destination's own carve-out. checkIsCardFolder closes the hole
where a lane's notes.txt would have been relocated: card depth is
exact, UUID under UUID.

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Fixtures/README.md

@@ -15,7 +15,7 @@ Lane/card folder names are fixed literal lowercase-UUIDv4-shaped strings (never
 | `rich-board.kanban` | A full-breadth well-formed board: 2 lanes, 3 cards, bodies, styling (background/icon/iconColor/width), unknown + reserved frontmatter keys, `attachments/` and `comments/` with real content. Its `attachments/` also carries all four listing shapes — two ordinary files, a hidden one, and a subfolder with a file — so `Card.attachments`' flat rule (01-storage-format.md § Attachments) is asserted against a real tree. Also the board every `index.md` in the tree is round-tripped against. |
 | `interrupted-create.kanban` | The motivating skip-not-error case: a UUID-shaped lane folder and a UUID-shaped card folder, each with no `index.md` yet (folder created, write not yet landed). |
 | `non-uuid-strays.kanban` | Non-UUID-shaped folders at both lane and card depth, with and without `index.md` — name shape gates candidacy before the file is ever read. |
-| `stray-files.kanban` | Stray (non-directory) files at board, lane, and card level — never level candidates, never warned about. |
+| `stray-files.kanban` | Stray (non-directory) files at board, lane, and card level — never level candidates, never warned about. The **card-level** one (`scratch.md`) is also the loose-file carve-out's golden case: tolerated everywhere else, it is reported in `LoadResult.looseCardFiles` for the app to relocate into `attachments/` (01-storage-format.md § Fractal layout ▸ Rules, settled 2026-07-28). Detection is read-only, so the file stays put on disk. |
 | `tombstones.kanban` | A tombstoned lane and a tombstoned card, both still on disk and still in the snapshot, flagged (`isDeleted`) rather than removed. Also proves a tombstoned lane doesn't recursively flag its own un-deleted children. |
 | `duplicate-order-tie-break.kanban` | Three cards sharing one `order` in one lane, and two lanes sharing one `order` — both broken by folder name, ascending. |
 | `unknown-key-order.kanban` | Unknown/reserved frontmatter keys interleaved with schema-owned ones at board, lane, and card level — `document.unknownFields` must preserve exactly the order they were written in. |