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.

CHANGELOG.md

@@ -14,6 +14,9 @@ All notable changes to `ltk` are documented here. The format is based on [Keep a
 - **Responsive sizing system** with two selectable modes via `WidgetScaling` (`Fluid` / `Physical`; `set_widget_scaling` / `widget_scaling`, default `Fluid`). New `Length` constructors — `orient( portrait, landscape )` (a percentage of the width in portrait, of the height in landscape), `fluid( px )` (surface-proportional, calibrated against `set_fluid_reference` and bounded by `FLUID_MIN` / `FLUID_MAX`), `dp( px )` (constant physical size scaled by `set_density` / `density`), and `widget( px )` (picks fluid or dp per the active mode). `Canvas::geom_px` (geometry, physical layout space) and `Canvas::font_px` (font, bridging the logical / physical split per mode) give widgets and apps one resolution path.
 - **`Button::font_size` / `height` / `width`** and **`TextEdit::height`** builders, all `impl Into<Length>`, so control boxes scale with the surface. `Text::line_height( mult )` opens the gap between wrapped lines. `Separator::pad_v` (with `Length::px( 0.0 )` for a flush divider).
 - **Performance guardrails**: opt-in diagnostics via `LTK_PERF_WARN=1` (stuck animation, sustained software-render animation, low `poll_interval`) and a ~30 Hz software-animation cap overridable with `App::cap_software_animation`.
+- **`grid_min_cell( width )` and `WrapGrid::max_columns( n )`** — adaptive grid: the column count is derived at layout time from the available width so every cell is at least `width` wide (any `Length`; never fewer than one column), re-derived on every resize; `max_columns` caps the count so cells grow instead of multiplying on wide surfaces. `grid( n )` keeps the fixed-count behaviour.
+- **Vertical flex: `flex( child )` now distributes leftover height inside a `Column`**, mirroring its leftover-width behaviour in a `Row` — weights split the spare space between flex / spacer siblings, the child draws inside the allocated share, and it contributes zero to the column's natural height like a weight-only spacer.
+- **`make stylecheck` / `scripts/style-check.sh`** — mechanical checks for the grep-verifiable subset of the style guide (tab indentation, spaces inside attribute brackets), wired into CI. The whole tree was normalized to pass (82 attribute sites).
 - **`ltk::orientation()` / `viewport_size()` / `set_viewport_size`** and the `Orientation` enum — the runtime records the main surface's physical dimensions on every configure, so `view()` can branch a layout on portrait vs landscape (`match ltk::orientation() { … }`) without tracking `on_resize` by hand; the portrait/landscape rule matches `Length::orient` (square counts as portrait). Embedders driving `core::UiSurface` directly call `set_viewport_size` themselves. `examples/clip_path.rs` demonstrates it.
 - **`test-support` Cargo feature** gates the `test_support` module so third-party builds never see it (ltk's own `make test` enables it).