Slice 10 d1: RCSS animation — dock slide-in + per-slot settle

Layers animation on c2's static pipeline with zero data-model / contract change
(c2 glue test passes unchanged). The dock body slides in from the left edge on
empty->non-empty via an RCSS transition on transform:translateX driven by a bound
`open` bool (data-class-open); slide-out defers set_visible(false) until the
slide's transitionend so the close is seen. Each freshly instanced preview card
plays a one-shot @keyframes (scale 0.72->1.0 + fade, transform-origin top-left =
"scales down into a spot"). Dock stays a 240px left-edge strip (animation is a
transform inside the surface, never a resize) so windows stay clickable.

Key finding: RmlUi 6.2 dispatches animationend/transitionend as real events, and
the substrate's data-event-* path binds a listener for ANY registered event — so
data-event-transitionend routes into bind_event with NO kernel change. The ONLY
remaining gap for the literal cross-screen window->dock flight is a single
input-transparent UiSurfaceSpec flag (a kernel boundary decision, NOT built).
Restore stays instant in d1 (animated grow-back is e1's drag-out).

10/10 suites green on build + build-asan (asan clean). Visual feel is real-seat.

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