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.

Makefile

@@ -5,7 +5,7 @@ DOCDIR   ?= /usr/share/doc/libltk-doc/html
 # ignore filesystem entries with the same name — without this `examples`
 # silently no-ops because the `examples/` directory exists, and `doc`
 # would do the same once `target/doc` is around.
-.PHONY: all test doctest-md audit doc install examples clean distclean
+.PHONY: all test doctest-md stylecheck audit doc install examples clean distclean
 
 all:
 	cargo build --release
@@ -20,6 +20,11 @@ test:
 doctest-md:
 	./scripts/doctest-md.sh
 
+# Mechanical style checks (tabs, attribute-bracket spacing) — the
+# grep-verifiable subset of code_style_guide.md.
+stylecheck:
+	./scripts/style-check.sh
+
 audit:
 	@command -v cargo-audit >/dev/null 2>&1 || cargo install cargo-audit --locked
 	cargo audit