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

KanbanTests/MotionTests.swift

@@ -111,6 +111,16 @@ struct MotionCurveTests {
         #expect(Motion.laneResizeWindowDuration == 0.2)
         #expect(Motion.laneResize(reduced: false) == .snappy(duration: Motion.laneResizeWindowDuration))
     }
+
+    /// **The carousel expansion is the structural voice** (03-board-ui.md § Motion's first named
+    /// narrow key). What the change *is* is positional — the card grows and its column-mates slide
+    /// down — so it takes the general snappy spring rather than the filtering one. Pinned as an
+    /// identity with `structural` rather than as a literal, because if the two ever diverge it
+    /// should be a deliberate edit to one line.
+    @Test func theCarouselExpansionIsTheStructuralVoice() {
+        #expect(Motion.carouselExpansion(reduced: false) == Motion.structural(reduced: false))
+        #expect(Motion.carouselExpansion(reduced: false) != Motion.contentReflow(reduced: false))
+    }
 }
 
 // MARK: - Reduce Motion
@@ -125,6 +135,7 @@ struct ReduceMotionVariantTests {
         #expect(Motion.delete(reduced: true) == nil)
         #expect(Motion.laneResize(reduced: true) == nil)
         #expect(Motion.contentReflow(reduced: true) == nil)
+        #expect(Motion.carouselExpansion(reduced: true) == nil)
     }
 
     /// "Appear/disappear is scale + fade (cards scale from ~0.8, lanes ~0.9, combined with opacity)."
@@ -139,4 +150,14 @@ struct ReduceMotionVariantTests {
         #expect(Motion.cardAppearance(reduced: true) == .crossfade)
         #expect(Motion.laneAppearance(reduced: true) == .crossfade)
     }
+
+    /// **The carousel is the one appearance with no reduced variant, because it is already the
+    /// reduced form.** 03 fixes an appear/disappear scale for cards and lanes and for nothing else,
+    /// and the carousel needs none: the movement is the card growing, which `carouselExpansion`
+    /// animates and Reduce Motion makes instant, so the content only has to fade in over it. This
+    /// asserts the parameterless accessor is deliberate rather than an omission.
+    @Test func theCarouselAppearanceIsTheCrossfadeInBothVariants() {
+        #expect(Motion.carouselAppearance == .crossfade)
+        #expect(Motion.carouselAppearance != Motion.cardAppearance(reduced: false))
+    }
 }