Build the sole-selection attachment carousel

The card face's in-place expansion (03-board-ui.md § Card face): a card
with attachments, as the sole selection, grows a paged media band below
its title; every other state stays compact behind the quiet paperclip.

- CardCarousel owns the pure rules: the sole-selection predicate decides
  by identity (a sole-selected lane matches no face, no snapshot walk),
  and expansion is suppressed outside the animation key while a rubber
  band is active — a band names a set in progress, so carousels neither
  flicker nor animate under it.
- QuickLook thumbnails generate off-main into a per-window cache keyed to
  survive reselection, with the Finder-icon fallback while loading and
  for non-previewable types; pages ride the platform paging behavior,
  dots (glass underlay, solid under Reduce Transparency) click to page,
  and a local wheel monitor turns a discrete tick into one clamped page —
  precise trackpad pans fall through untouched.
- The expansion animates under Motion's new carouselExpansion transaction
  keyed narrowly on the sole-selected card; Reduce Motion goes instant.
  Drop-slot math and the marquee read the expanded height for free — both
  re-register on every size change.

928 unit tests (24 new). m5-interactions complete.

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

Kanban/UI/Board/BoardView.swift

@@ -94,6 +94,14 @@ struct BoardView: View {
     /// the cards and trash rows only *register* into it (`MarqueeTargetRegistry`).
     @State private var marqueeTargets = MarqueeTargetRegistry()
 
+    /// This window's attachment thumbnails, for the sole-selected card's carousel (03-board-ui.md
+    /// § Card face). `@State` for the sessions' reason — one per window, living exactly as long as
+    /// the window, so a board that closes forgets the files it was showing (see
+    /// `AttachmentThumbnailCache` on why the window and not the app). Handed down through the
+    /// environment rather than threaded, since nothing between here and a card face has any use for
+    /// it.
+    @State private var thumbnails = AttachmentThumbnailCache()
+
     /// The name of the strip's coordinate space, which is what a drop out of the trash is resolved
     /// in: `LaneLayoutMath.laneIndex` reads an x measured from the strip's leading edge, outer margin
     /// included, and no global or lane-local space is that.
@@ -149,6 +157,7 @@ struct BoardView: View {
             .onDrop(of: boardDropTypes, delegate: StripDropDelegate(context: dropContext))
         }
         .background(boardBackground)
+        .environment(thumbnails)
         // The window-level fallback, *behind* the specific targets: a release over any in-window
         // region they don't cover commits the current proposal instead of leaking the session into a
         // cancel-snapback — the drop lands where the shadows show, which is what the shadows promise.