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.

code_style_guide.md

@@ -133,4 +133,4 @@ fn main()
 - The item brace styles (`brace_style`, `control_brace_style`) are **unstable, nightly-only** options.
 - Even on nightly, the combination of Allman opening braces with a compact `} else {` is not representable: `control_brace_style = "AlwaysNextLine"` also pushes `else` onto its own line.
 
-The repository ships a `rustfmt.toml` containing only `disable_all_formatting = true`, so an accidental `cargo fmt` — or an editor with format-on-save wired to rustfmt — is a no-op instead of a 40 000-line diff. Style is enforced by review, not by a formatter.
+The repository ships a `rustfmt.toml` containing only `disable_all_formatting = true`, so an accidental `cargo fmt` — or an editor with format-on-save wired to rustfmt — is a no-op instead of a 40 000-line diff. The mechanically verifiable subset (tab indentation, attribute-bracket spacing) is enforced by `make stylecheck` (`scripts/style-check.sh`), which CI runs on every push; everything else — brace placement, paren spacing, naming — is enforced by review.