Drop semantics under mid-drag foreign reloads

Fills the gap: resting zones recompute per snapshot (frozen inputs are
the dragged items' sizes and the pointer); proposals re-validate by
liveness with release-cancels-when-invalid; an emptied drag cancels
itself, partial vanishing drops survivors.

Claude-Session: https://claude.ai/code/session_01HJ7PhFNmQ19bvy9RMD6GSb

DESIGN/04-interactions.md

@@ -19,6 +19,7 @@ Selection, drag & drop, keyboard, clipboard, search. This is where the old app s
 - **Cross-board copy** (the default): cards and lanes (including multi-selections) drag between open boards; fresh-GUID duplicates land at the drop, originals stay, `created` is kept (a copy is a fork — 01-storage-format.md). Lanes copy cards and all — transferring workflow structure between boards is safe by default. A lane copy **strips tombstoned cards**: the copy transfers content, and trash isn't content (09-templates.md's instantiation precedent — a board isn't born with trash); the tombstoned originals stay recoverable in the source board. A ⌘-drag *move* carries them whole — the folder moves as-is, and they land in the destination's trash.
 - **Cross-board move** (⌘-drag): a real filesystem move, works across volumes — identity travels. A moved folder whose UUID already exists in the destination board arrives as a fresh-UUID copy (01-storage-format.md's import-boundary rule); in a compound move (lane with cards, multi-selection) only the colliding folders are reminted — the rest is a true move (01's per-folder degradation).
 - **Files from Finder**: dropped on a card → copied into its `attachments/` (any type, multi-file; card highlights while hovered). Dropped on lane empty space → creates a card with the file attached, titled with the filename without its extension (multi-file drop: one card per file).
+- **A foreign reload mid-drag re-grounds the drag, never corrupts the drop** (settled — a two-second drag racing agent edits is the designed concurrency). Three rules compose: (1) **geometry re-derives** — the frozen-at-drag-start inputs are the *dragged items'* sizes and the physical pointer only (03-board-ui.md ▸ Motion); the analytic resting zones recompute against each new snapshot, so a foreign lane-count re-divide mid-drag just moves the zones and the next proposal targets the board as it now is. (2) **Proposals re-validate by liveness** — a proposal whose target lane was tombstoned or vanished in the reload is invalidated (tombstoned lanes are never drop targets — The trash below); the shadow withdraws and no proposal stands until the pointer reaches a live target, and **release with no valid proposal cancels** — items return, nothing is written; a card is never filed under a `deleted:` parent. (3) **An emptied drag cancels itself** — drag membership is already a UUID set that vanished items leave silently (02-architecture.md); when the *last* dragged item vanishes the replica dissolves and release is a no-op. Partial vanishing drops the survivors, matching the pending-cut precedent.
 
 ## Clipboard