Forge's app switcher needs two layouts the existing widget set didn't cover. The desktop grid wants a partial last row centred under the rows above (3 tiles → row 1: two, row 2: one centred) so a 7-of-9 leftover band reads balanced rather than left-aligned. The mobile variant wants a horizontal carousel where the focused tile sits centred in the viewport at a configurable fraction of its width and its neighbours peek out on the sides at a fixed gap.
Extend `WrapGrid` with `centre_last_row( bool )`. When set, layout offsets a row that has fewer than `columns` children by `(missing * (cell_w + spacing)) / 2` so it stays centred inside the content rect. Defaults to false; every existing call site continues to land tiles flush-left. Covered by three layout tests (centred partial row, full row no-op, off-by-default).
Add the `Carousel` widget at `src/widget/carousel/`. It is a pure layout primitive: `focused_width_frac` (0.05–1.0, clamped), `gap` and `offset` are owned by the caller, leaving drag / inertia / snap policy to the host so the compositor can plug in its existing touch pipeline. Each child gets a rect at `base_x + idx * (child_w + gap)` and the full viewport height; `snap_offset( viewport_w, idx )` translates index to centring offset and `focused_index( viewport_w )` rounds the current offset back to the nearest tile. Plumbed into `Element::Carousel` with the matching arms in `widget/element.rs` and walker in `draw/layout.rs`; re-exported as `ltk::{ Carousel, carousel }`. Covered by nine unit tests (layout, offset shift, snap / focus round-trip, frac clamp, child height) plus a `cargo run --example carousel` demo with Prev / Next / arrow-key navigation against an external offset state. The example is wired into the `examples` Makefile target.
Updates the widget catalogue and the `widget/mod.rs` landing comment to list the carousel under "Clipping wrappers" and to mention `centre_last_row` in the grid section.
