layout: adaptive grid and vertical flex; enforce the mechanical style rules

grid_min_cell( width ) adds the adaptive mode WrapGrid was missing: instead of a fixed column count, the count is derived at layout time from the available width so every cell is at least `width` wide (any Length, so a fluid threshold works; never fewer than one column), re-derived on every resize. Cells then share the width equally, and WrapGrid::max_columns( n ) caps the derived count so cells grow instead of multiplying on very wide surfaces — the cap only applies to the adaptive mode, grid( n ) keeps the fixed behaviour untouched. Four new layout tests cover width derivation, spacing accounting, the one-column floor and the cap; the row-wrap assertions check X positions rather than Y because zero-height spacer children produce zero-height rows.
flex( child ) now distributes leftover height inside a Column, mirroring its leftover-width behaviour in a Row: flex children join the weight pool alongside weight-only spacers and y-scrolls, draw their child inside the allocated share at full inner width, and contribute zero to the column's natural height exactly like a row counts flex width. This closes the documented "vertical flex is not yet implemented" gap; the Flex rustdoc and the widgets.md entry now describe both axes.
Add scripts/style-check.sh and a make stylecheck target, wired into CI: mechanical verification of the grep-checkable subset of code_style_guide.md — tab indentation and spaces inside attribute brackets — with comment lines skipped so prose may cite raw attribute syntax. To turn the check on green, the 82 unspaced attribute sites across 24 files (#[test], #[derive(...)], #[cfg(...)], #[allow(...)]) were normalized to the spaced form. CONTRIBUTING and the style guide reference the new target; brace placement and paren spacing remain review concerns.

src/event_loop/app_data.rs

@@ -56,7 +56,7 @@ pub struct AppData<A: App>
 	/// failed to initialise or `LTK_FORCE_SOFTWARE=1` — every surface then
 	/// falls back to the SHM path.
 	pub egl_context:        Option<Arc<EglContext>>,
-	#[allow(dead_code)]
+	#[ allow( dead_code ) ]
 	pub xdg_shell:          Option<XdgShell>,
 	/// Shared layer-shell binding used for the main surface and every
 	/// overlay. `None` when the compositor does not advertise the protocol.
@@ -289,7 +289,7 @@ impl<A: App> AppData<A>
 	/// refers to an overlay that is not currently registered — callers must
 	/// only pass focus values obtained from [`focus_for_surface`] or from the
 	/// per-device focus fields, which the run loop keeps in sync.
-	#[allow( dead_code )]
+	#[ allow( dead_code ) ]
 	pub( crate ) fn surface( &self, focus: SurfaceFocus ) -> &SurfaceState<A::Message>
 	{
 		match focus
@@ -313,7 +313,7 @@ impl<A: App> AppData<A>
 	}
 
 	/// Mutable counterpart of [`surface`].
-	#[allow( dead_code )]
+	#[ allow( dead_code ) ]
 	pub( crate ) fn surface_mut( &mut self, focus: SurfaceFocus ) -> &mut SurfaceState<A::Message>
 	{
 		match focus