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/widget/separator/tests.rs

@@ -3,7 +3,7 @@
 
 use super::*;
 
-#[test]
+#[ test ]
 fn default_thickness_and_pad_follow_the_mode()
 {
 	let s = separator();
@@ -11,7 +11,7 @@ fn default_thickness_and_pad_follow_the_mode()
 	assert_eq!( s.pad_v,     None );
 }
 
-#[test]
+#[ test ]
 fn explicit_zero_pad_v_is_flush_not_the_mode_default()
 {
 	// The Option repr lets `0.0` mean a real flush divider, distinct from
@@ -22,7 +22,7 @@ fn explicit_zero_pad_v_is_flush_not_the_mode_default()
 	assert_eq!( s.preferred_size( 200.0, &canvas ).1, canvas.geom_px( theme::THICKNESS ) );
 }
 
-#[test]
+#[ test ]
 fn preferred_height_includes_padding()
 {
 	let _g = crate::TEST_GLOBALS_LOCK.lock().unwrap_or_else( |e| e.into_inner() );