Implement the motion language

One named home for every curve and duration — the two-voice split 03
fixes (snappy structural: drag reflow 0.18, delete 0.25, lane resize
0.2; smooth content-reflow 0.28), the scale-and-fade appear/disappear
transitions (cards 0.8, lanes 0.9), and a Reduce Motion variant on
every accessor (animations go instant, transitions go crossfade). A
post-migration grep holds the invariant: zero motion literals outside
Motion.swift. The animate-vs-snap split lands where Lanework's
one-way flow puts it: the store's reload seam. An app-mediated echo
applies its snapshot inside the structural transaction; foreign and
reconciling reloads — and every bracket-ending wholesale reload —
assign bare, because live-reload is the board becoming what's on
disk, not an event to perform. A window that merged foreign events
into an app-mediated span animates, deliberately: the ratified merge
rule makes it indistinguishable from a pure echo, and a test pins
that reading so a future mixed case fails loudly. The lane-reorder
reflow keys on the drop proposal alone (pointer tracking stays 1:1),
the resize session freezes its Reduce Motion answer at drag start so
the unit tick and the window resize can never disagree, and the m5
drag / m5 search / m7 undo voices are named seams waiting for their
call sites. 13 new tests.

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

Kanban/UI/Board/LaneView.swift

@@ -69,6 +69,10 @@ struct LaneView: View {
     /// business knowing about `WindowGroup` keys.
     let openCard: (ItemID) -> Void
 
+    /// Reduce Motion, for the card transition below (10-accessibility.md). Read from the environment
+    /// and handed to `Motion`, which owns what "reduced" means.
+    @Environment(\.accessibilityReduceMotion) private var reduceMotion
+
     /// Spacing between cards, and between the interior columns.
     private let cardSpacing: CGFloat = 8
 
@@ -305,12 +309,21 @@ struct LaneView: View {
             // `units` columns of `standard` (03-board-ui.md § Layout — full visibility).
             MasonryLayout(columns: columns, spacing: cardSpacing) {
                 ForEach(slots) { slot in
-                    switch slot {
-                    case let .card(card):
-                        CardFaceView(store: store, card: card, openCard: openCard)
-                    case .placeholder:
-                        NewCardStubView(store: store, openCard: openCard)
+                    Group {
+                        switch slot {
+                        case let .card(card):
+                            CardFaceView(store: store, card: card, openCard: openCard)
+                        case .placeholder:
+                            NewCardStubView(store: store, openCard: openCard)
+                        }
                     }
+                    // "Appear/disappear is scale + fade (cards scale from ~0.8 …)"
+                    // (03-board-ui.md § Motion), which is how a create, a delete, a Put Back and
+                    // (m5) a search filter's leavers all reach the masonry. The placeholder wears it
+                    // too: it is the card, one round trip early. Whether any of it *performs* is
+                    // decided upstream — at the reload for the real cards (`Motion.reloadAnimates`),
+                    // at the gesture for the placeholder, which touches no disk.
+                    .transition(Motion.cardTransition(reduced: reduceMotion))
                 }
             }
             .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .topLeading)