Name the drop proposal as drag reflow's animation key

Resolves 03 ▸ Motion's self-conflict: narrow keys are the sole-selected
card, the search query, and the drag's drop proposal (reflow-to-make-
room animates under it); animation-free means the replica and marquee's
1:1 cursor tracking and multi-select churn, not reflow.

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

DESIGN/03-board-ui.md

@@ -100,7 +100,7 @@ The pathfinder's animation behavior carries over as the committed motion languag
 - **Equivalent operations share one dialect.** Paste animates exactly like a drop commit (same curve, same duration) so the clipboard's move story *feels* like drag landing; keyboard one-slot moves slide for the same reason a drop does — an item that teleports is harder to follow than one that slides; cut dims the card in place, Finder-style, until paste moves it (04-interactions.md).
 - **Appear/disappear is scale + fade** (cards scale from ~0.8, lanes ~0.9, combined with opacity). A restore that moves a card across lanes flies it from old frame to new via matched geometry. Search-hiding rides the same structural transition — hiding is removal, not a special fade.
 - **Some things deliberately never animate**: the rubber-band marquee tracks the cursor 1:1 (an eased band visibly lags the mouse), and the selection highlight rides whatever transaction is active rather than easing on its own.
-- **Animated transactions are keyed narrowly** — on the sole-selected card, on the search query — never on broad state, so multi-select churn, marquee drags, and in-flight drags stay animation-free by construction rather than by suppression.
+- **Animated transactions are keyed narrowly** — on the sole-selected card (carousel expansion), on the search query (filter reflow), on the drag's **drop proposal** (the reflow-to-make-room above animates under it, ~0.18 s) — never on broad state like the selection set. What stays animation-free by construction rather than by suppression: the drag replica and the marquee rectangle (1:1 cursor tracking — animating input echo would be lag), and multi-select churn.
 - **Motion never feeds back into logic** (the pathfinder's animation-proof-inputs rule, kept as a hard constraint): drop-proposal math reads analytically computed resting zones, the physical mouse position, and item sizes frozen at drag start — never mid-flight measured frames, which are garbage precisely during the ~0.2 s reflow they trigger.
 - **Reduce Motion is a rewrite obligation, not an inheritance**: the pathfinder ships zero reduced variants; 10-accessibility.md's commitments (crossfade or instant for reflow, search animate-out, the drag replica, trash) are new work.