add Carousel widget and WrapGrid::centre_last_row

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.

src/layout/wrap_grid.rs

@@ -32,15 +32,18 @@ use crate::widget::Element;
 pub struct WrapGrid<Msg: Clone>
 {
 	/// Child widgets laid out in row-major order.
-	pub children:  Vec<Element<Msg>>,
+	pub children:        Vec<Element<Msg>>,
 	/// Number of columns per row.
-	pub columns:   usize,
+	pub columns:         usize,
 	/// Horizontal gap between cells (pixels).
-	pub spacing_x: f32,
+	pub spacing_x:       f32,
 	/// Vertical gap between rows (pixels).
-	pub spacing_y: f32,
+	pub spacing_y:       f32,
 	/// Padding on all sides (pixels).
-	pub padding:   f32,
+	pub padding:         f32,
+	/// When `true`, a partial last row is centred horizontally within
+	/// the grid's content rect instead of being left-aligned.
+	pub centre_last_row: bool,
 }
 
 impl<Msg: Clone> WrapGrid<Msg>
@@ -81,6 +84,14 @@ impl<Msg: Clone> WrapGrid<Msg>
 		self
 	}
 
+	/// Centre a partial last row horizontally inside the content rect.
+	/// Default is `false` (left-aligned, like other grids).
+	pub fn centre_last_row( mut self, yes: bool ) -> Self
+	{
+		self.centre_last_row = yes;
+		self
+	}
+
 	/// Compute the preferred size given an available width.
 	pub fn preferred_size( &self, max_width: f32, canvas: &Canvas ) -> (f32, f32)
 	{
@@ -133,9 +144,16 @@ impl<Msg: Clone> WrapGrid<Msg>
 				.map( |c| c.preferred_size( cell_w, canvas ).1 )
 				.fold( 0.0_f32, f32::max );
 
-			for col in 0..(end - start)
+			let items_in_row = end - start;
+			let row_offset   = if self.centre_last_row && items_in_row < cols
 			{
-				let x    = x0 + col as f32 * (cell_w + self.spacing_x);
+				let missing = (cols - items_in_row) as f32;
+				missing * (cell_w + self.spacing_x) / 2.0
+			} else { 0.0 };
+
+			for col in 0..items_in_row
+			{
+				let x    = x0 + row_offset + col as f32 * (cell_w + self.spacing_x);
 				let crect = Rect { x, y, width: cell_w, height: row_h };
 				out.push( ( crect, start + col ) );
 			}
@@ -151,11 +169,12 @@ impl<Msg: Clone> WrapGrid<Msg>
 	{
 		WrapGrid
 		{
-			children:  self.children.into_iter().map( |c| c.map_arc( f ) ).collect(),
-			columns:   self.columns,
-			spacing_x: self.spacing_x,
-			spacing_y: self.spacing_y,
-			padding:   self.padding,
+			children:        self.children.into_iter().map( |c| c.map_arc( f ) ).collect(),
+			columns:         self.columns,
+			spacing_x:       self.spacing_x,
+			spacing_y:       self.spacing_y,
+			padding:         self.padding,
+			centre_last_row: self.centre_last_row,
 		}
 	}
 }
@@ -320,6 +339,43 @@ mod tests
 		assert!( (rects[0].0.x - 50.0).abs() < 0.01 );
 		assert!( (rects[0].0.y - 30.0).abs() < 0.01 );
 	}
+
+	// --- layout: centre_last_row ---
+
+	#[test]
+	fn last_row_centred_when_partial()
+	{
+		// 3 children, 2 cols => row 0: 2 items, row 1: 1 item centred.
+		// cell_w = 200/2 = 100; centred-offset = (2-1)*100/2 = 50.
+		let g = spacer_grid( 2, 3, 0.0, 0.0 ).centre_last_row( true );
+		let c = canvas();
+		let rect = Rect { x: 0.0, y: 0.0, width: 200.0, height: 400.0 };
+		let rects = g.layout( rect, &c );
+		assert!( (rects[2].0.x - 50.0).abs() < 0.01 );
+	}
+
+	#[test]
+	fn centre_last_row_noop_on_full_row()
+	{
+		// 4 children, 2 cols => both rows full; nothing to centre.
+		let g = spacer_grid( 2, 4, 0.0, 0.0 ).centre_last_row( true );
+		let c = canvas();
+		let rect = Rect { x: 0.0, y: 0.0, width: 200.0, height: 400.0 };
+		let rects = g.layout( rect, &c );
+		assert!( rects[2].0.x.abs() < 0.01 );
+		assert!( (rects[3].0.x - 100.0).abs() < 0.01 );
+	}
+
+	#[test]
+	fn centre_last_row_off_by_default()
+	{
+		// Same case as above but without the flag — last item stays at x=0.
+		let g = spacer_grid( 2, 3, 0.0, 0.0 );
+		let c = canvas();
+		let rect = Rect { x: 0.0, y: 0.0, width: 200.0, height: 400.0 };
+		let rects = g.layout( rect, &c );
+		assert!( rects[2].0.x.abs() < 0.01 );
+	}
 }
 
 /// Create a grid layout with the given number of columns.
@@ -338,10 +394,11 @@ pub fn grid<Msg: Clone>( columns: usize ) -> WrapGrid<Msg>
 {
 	WrapGrid
 	{
-		children:  Vec::new(),
+		children:        Vec::new(),
 		columns,
-		spacing_x: 8.0,
-		spacing_y: 8.0,
-		padding:   0.0,
+		spacing_x:       8.0,
+		spacing_y:       8.0,
+		padding:         0.0,
+		centre_last_row: false,
 	}
 }