text_edit font sizing on Length; add Button::width and Text::line_height

Unify TextEdit's font size with the button label. The button resolved its `font_size` Length in font space (`viewport_logical`, so the `× dpi_scale` at raster lands the right physical size), while `TextEdit::font_size` took a raw `f32` that the draw / hit-test paths treated as a logical size. A caller that resolved a Length against the physical surface and passed the result as that `f32` therefore double-counted `dpi_scale` and got a font that rendered too large. `TextEdit::font_size` now takes `impl Into<Length>` and is resolved against `viewport_logical` exactly like the button, so the two paths agree. The field stores `Option<Length>` (`None` follows the widget-scaling mode at the theme default), retiring the old `f32` sentinel (`0.0` = mode, negative = fluid design px). `font_size_fluid` becomes shorthand for `Length::fluid( n )`. The `Option<Length>` flows through the `WidgetHandlers::TextEdit` snapshot and `text_input_geometry` and is resolved at draw / hit-test time, both of which carry a canvas; the inner measure helpers (`wrapping`, `hit_test`) keep `f32` because they receive the already-resolved value. Backward compatible: `f32` call sites still compile via `f32: Into<Length>` (→ `Length::px`), with the same result as before.
Add `Button::width( impl Into<Length> )`. Text buttons size to their label plus padding; some layouts need a pinned width instead — a full-width or surface-proportional button. The new builder mirrors `height`: resolved in physical layout space, clamped to the available `max_width`, propagated through `map_msg`, and a no-op for icon buttons.
Add `Text::line_height( mult )`. Wrapped multi-line text used the font's declared leading (`new_line_size`), which is tight for some labels; the multiplier scales the gap between wrapped lines (`1.0`, the default, keeps the natural leading — every other `text` is unchanged — and a `0.5` floor keeps lines from overlapping). Applied uniformly in `preferred_size` and `draw` so the reported height and the drawn baselines stay consistent.
Tests: `button` gains a pinned-width and a size-to-content case; `text` gains a line-height default / clamp test and a check that doubling the line height doubles a wrapped block's reported height. Docs: `docs/widgets.md` `text` / `button` / `separator` sections updated for the new builders, and a `CHANGELOG.md` "Unreleased" section covering this batch alongside the responsive work already landed.

CHANGELOG.md

@@ -2,6 +2,22 @@
 
 All notable changes to `ltk` are documented here. The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/), and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Added
+
+- **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`.
+- **`test-support` Cargo feature** gates the `test_support` module so third-party builds never see it (ltk's own `make test` enables it).
+
+### Changed
+
+- **`OverlaySpec::size`** is now `( Length, Length )` (was `( u32, u32 )`), resolved against the main surface when the overlay is materialized; wrap existing sizes in `Length::px( … )` for the old fixed behaviour.
+- **`TextEdit::font_size`** and **`Separator::thickness` / `pad_v`** now take `impl Into<Length>` (were `f32`), resolved like the button label (font space) / geometry space; the `f32` sentinels are gone (`f32` call sites still compile via `Into<Length>`).
+- **Renamed** `set_design_reference` / `design_reference` → `set_fluid_reference` / `fluid_reference`. **`Length::dp` changed meaning** — it used to be a surface-proportional value, and that behaviour now lives on `Length::fluid`; `dp` is the constant-physical-size unit.
+- **Widget struct fields are now `pub( crate )`** (configured through builders), except the value / state types apps read or construct (`Time`, `Date`, `ComboState`).
+
 ## [0.2.0] — 2026-06-25
 
 This release adds the primitives an embedder needs to drive ltk as the render backend for a retained, externally-owned widget tree (for example projecting an Android view hierarchy onto an ltk surface). Each is kept general rather than tied to one consumer.