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/surface.rs

@@ -34,18 +34,18 @@ use crate::widget::LaidOutWidget;
 /// `Main` refers to the application's main surface (xdg window or layer
 /// shell). `Overlay( id )` refers to an auxiliary layer-shell surface created
 /// from an entry in [`crate::app::App::overlays`].
-#[derive( Debug, Clone, Copy, PartialEq, Eq, Hash )]
+#[ derive( Debug, Clone, Copy, PartialEq, Eq, Hash ) ]
 pub( crate ) enum SurfaceFocus
 {
 	Main,
-	#[allow( dead_code )]
+	#[ allow( dead_code ) ]
 	Overlay( OverlayId ),
 }
 
 /// Configuration for a layer-shell surface, used both for the main surface
 /// (when the app uses [`crate::app::ShellMode::Layer`]) and for each overlay
 /// returned by [`crate::app::App::overlays`].
-#[derive( Clone )]
+#[ derive( Clone ) ]
 pub( crate ) struct LayerConfig
 {
 	pub layer:              Layer,