Implement Finder file drops

Files from Finder land on the board per 04-interactions.md § Drag & drop:

- Dropped on a card, they copy into its attachments/ (any type, multi-file),
  the face highlighting while hovered; the hovered card is resolved by
  hit-testing the same analytic masonry frames the card zones are built
  from, so attach-beats-create adds no drop region and cannot drift from
  the dispatch.
- Dropped on lane empty space, one card per file — filename minus extension
  as the title (a blank stem omits the key), fresh GUID, rank at the drop
  position through the ordinary insertion machinery, the file attached —
  all in one bracket; a failed import removes the just-minted card, so
  creating-then-abandoning never leaves an empty card behind.
- Every board drop surface now declares .fileURL beside the two board
  types (the single-target-dispatch rule); tombstoned surfaces are inert;
  file sessions ride a distinct session mode with their own watchdog and
  no hysteresis, leaving the board-drag machinery untouched.
- Also: four empty fixture directories pinned with .keep files so git
  preserves them, and a test-only visibility fix in DragSessionTests.

811 unit tests (12 new).

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

README.md

@@ -16,7 +16,7 @@ Lanework is in early development. This list tracks what has actually shipped and
 - **Card faces** — a card reads as a leading SF Symbol, its title (or a quiet untitled placeholder), and a quiet paperclip when it has attachments — title-only by design, no body excerpt. Colour is an edge accent rather than a fill: `background` paints a stripe down the card's left edge and `iconColor` tints the symbol, both written as a kebab-case palette name (12 icon tints, 12 backgrounds) or a `#RRGGBB[AA]` hex. Everything degrades rather than complains — an unreadable colour simply doesn't paint, and the value stays on disk exactly as written. Each card's snapshot carries its attachment names, listed flat and in Finder order (top-level files only; subfolders, hidden files, and symlinks are preserved but never surfaced).
 - **Creating and renaming** — New Card (⌘N) files into the selected card's lane immediately after it, a selected lane's bottom, or the last-active lane, opening a focused pseudo-card that exists nowhere on disk until its title commits (Return commits and re-selects the lane, ⌘↩ also opens the card window, Escape or clicking away discards, and a failed create discards rather than waiting for a card that can't arrive). Inline rename — Return on a card, Board ▸ Rename for either kind — tracks its target by UUID, so a foreign move mid-edit is invisible and a tombstone or deletion discards the edit silently; committing empty removes the `title` key rather than writing a blank one. New Lane is ⇧⌘N. Every mutating command disables while an editor holds the keyboard and under the read-only lock.
 
-- **Drag & drop** — cards, lanes and trash rows all travel as real system drag sessions, so a drag crosses window boundaries, shows the system's own copy badge, and carries a full-size replica of what it picked up. A dashed shadow sits at the exact landing spot and the board reflows to make room; the proposal is pure geometry over an analytically reconstructed resting layout — never measured mid-animation frames — so the shadow is stable rather than jittery, and a lane only reflows once the cursor reaches where the dragged run would actually land, holding its last proposal across the ambiguous stretch in between. Dragging any member of a multi-selection drags the whole selection: N contiguous shadows, one insertion point, landing in flatten order. Locality picks the default the way Finder's volumes do — within a board a drag moves, between boards it copies, with ⌥ forcing copy and ⌘ forcing move and the badge tracking live as the cursor crosses a boundary; a lane reordering inside its own board ignores ⌥ entirely, and a lane copy strips tombstoned cards while a lane move carries them whole. Dragging a trash row onto a lane restores it at the drop position, ⌥ copies it out live instead, and dropping it on another board follows the same copy-out grammar. Lanes taller than their viewport autoscroll from either edge, re-resolving the landing spot on every step so a stationary cursor still lands where the shadow shows. A foreign edit mid-drag re-grounds the drag rather than corrupting the drop: the zones re-derive against each new snapshot, a proposal whose lane was deleted withdraws and a release with none simply cancels, and a drag whose items all vanish dissolves itself. At release the board keeps drawing the dropped arrangement until the write round-trips through the watcher, so nothing snaps back for a frame; every drop is one write bracket — one reload, one commit — whatever the set's size.
+- **Drag & drop** — cards, lanes and trash rows all travel as real system drag sessions, so a drag crosses window boundaries, shows the system's own copy badge, and carries a full-size replica of what it picked up. A dashed shadow sits at the exact landing spot and the board reflows to make room; the proposal is pure geometry over an analytically reconstructed resting layout — never measured mid-animation frames — so the shadow is stable rather than jittery, and a lane only reflows once the cursor reaches where the dragged run would actually land, holding its last proposal across the ambiguous stretch in between. Dragging any member of a multi-selection drags the whole selection: N contiguous shadows, one insertion point, landing in flatten order. Locality picks the default the way Finder's volumes do — within a board a drag moves, between boards it copies, with ⌥ forcing copy and ⌘ forcing move and the badge tracking live as the cursor crosses a boundary; a lane reordering inside its own board ignores ⌥ entirely, and a lane copy strips tombstoned cards while a lane move carries them whole. Dragging a trash row onto a lane restores it at the drop position, ⌥ copies it out live instead, and dropping it on another board follows the same copy-out grammar. Lanes taller than their viewport autoscroll from either edge, re-resolving the landing spot on every step so a stationary cursor still lands where the shadow shows. A foreign edit mid-drag re-grounds the drag rather than corrupting the drop: the zones re-derive against each new snapshot, a proposal whose lane was deleted withdraws and a release with none simply cancels, and a drag whose items all vanish dissolves itself. At release the board keeps drawing the dropped arrangement until the write round-trips through the watcher, so nothing snaps back for a frame; every drop is one write bracket — one reload, one commit — whatever the set's size. Files dragged in from Finder join the same dispatch: dropped on a card they copy into its `attachments/` (any type, multi-file, Finder-style renames on collision, the card highlighting while hovered), dropped on lane empty space they become one card per file — titled with the filename minus its extension, that file attached, landing at the drop position with a shadow per card. Tombstoned surfaces are inert to them, and a read-only board or an open inline editor refuses them outright.
 
 - **The keyboard** — the board is fully operable without the mouse, and every board function has a menu item. Arrows walk to the nearest card in the direction — across a lane's interior masonry columns, across lanes, and into and out of the shown trash — with ⇧-arrows extending the range from the anchor and stopping dead at the liveness and card/lane-entry boundaries rather than silently reaching past them. ⌥-arrows jump: ⌥↑/⌥↓ to the lane's first/last card, ⌥←/⌥→ to the first/last lane (⌥→ reaching the trash when it's shown), and a second ⌥↑ on a lane's first card escalates into selecting the lane itself — the one keyboard entry to lane selection, from which ←/→ move between lanes, ↓ descends back into the cards, and an empty selection seeds at the first lane's first card so an arrow from nothing always means the same thing. The selection scrolls itself into view. The Board menu carries the rest: Open Card (⌘↩) — the one command that stays live mid-edit, committing the title and opening the window — Move Up/Move Down (⌥⌘↑/⌥⌘↓), which sort within a lane in logical order and gather a scattered selection into a block behind its first card on the first press, and Move Left/Move Right (⌘←/⌘→), which slide a selected lane one slot and never into the trash. Deleting picks the successor sibling Finder-style, so repeated ⌫ walks down a lane; an external deletion deliberately doesn't.