responsive fluid/physical scaling, widget-API stabilization, and perf guardrails

Responsive scaling. ltk now offers two first-class ways to size a UI so it adapts across screens, chosen per process via `WidgetScaling { Fluid, Physical }` (`set_widget_scaling` / `widget_scaling`, default `Fluid`). Fluid sizing (`Length::fluid( px )`) makes a design pixel a proportion of the surface's smaller side, calibrated against a reference width (`set_fluid_reference` / `fluid_reference`, 412 px default) and bounded by `FLUID_MIN` / `FLUID_MAX`; physical sizing (`Length::dp( px )`) is a constant-physical-size pixel scaled by display density (`set_density` / `density`). `Length` gains `orient( portrait, landscape )` — resolve one value in portrait, another in landscape — plus `widget( px )`, which picks fluid or dp per the active mode. Canvas exposes `geom_px` (geometry, resolved in physical layout space) and `font_px` (font size, bridging logical / physical per mode) so widgets and apps share one resolution path. Note the rename: `set_design_reference` / `design_reference` became `set_fluid_reference` / `fluid_reference`, and `Length::dp` changed meaning — the old surface-proportional behaviour now lives on `Length::fluid`.
Widgets. Every stock widget resolves its default geometry and font through the widget-scaling mode instead of frozen pixels, so a whole UI scales coherently without per-call units. New size builders where they were missing: `button` gains `font_size` / `height`, `text_edit` gains `height` / `font_size_fluid`, `separator` gains `pad_v`, and assorted widgets accept a `Length` where they previously took only `f32`.
Overlays. `OverlaySpec::size` is now `( Length, Length )` instead of `( u32, u32 )`, resolved against the main surface when the overlay is materialized, so overlays can scale with the display; `Length::px( … )` reproduces the old fixed sizing.
API stabilization (toward 1.0). Widget struct fields are now `pub( crate )` — they are configured through builders, not field access — except the value / state types apps genuinely read or construct (`Time`, `Date`, `ComboState`), which stay public. The internal `test_support` helpers move behind a `test-support` Cargo feature (off by default, so third-party builds never see them; ltk's own `make test` enables it). `Separator` drops its `0.0`-means-mode sentinel for `Option<Length>`, so an explicit `pad_v( 0.0 )` is a real flush divider distinct from the mode-following default.
Performance guardrails. Opt-in diagnostics via `LTK_PERF_WARN=1` warn about stuck animations, sustained software-render animation, and low `poll_interval`; software-rendered animation is capped near 30 Hz to spare CPU on machines that fall back off EGL. Apps can override the cap with `App::cap_software_animation`.
Docs and build. The two scaling modes are documented in README, onboarding and architecture, with the earlier gradient / backdrop doc drift cleaned up. The Makefile now ships the `locales/` directory into the packaged crate (fixing i18n keys rendering raw for downstreams), builds the new `responsive` example, and runs tests with `--features test-support`.

src/widget/image/tests.rs

@@ -105,6 +105,26 @@ fn preferred_size_cover_and_default_match_for_uniform_scaling()
 	assert_eq!( normal.preferred_size( 200.0, &c ), covered.preferred_size( 200.0, &c ) );
 }
 
+#[ test ]
+fn short_side_sizes_by_width_in_portrait_keeping_aspect()
+{
+	// 200×100 source, portrait 480×900 → short side is the width.
+	// orient( 40, 5 ) portrait → 40 % of 480 = 192 width, height 192*(100/200)=96.
+	let img    = Image::new( solid_rgba( 200, 100 ), 200, 100 ).short_side( Length::orient( 40.0, 5.0 ) );
+	let canvas = Canvas::new( 480, 900 );
+	assert_eq!( img.preferred_size( 1000.0, &canvas ), ( 192.0, 96.0 ) );
+}
+
+#[ test ]
+fn short_side_sizes_by_height_in_landscape_keeping_aspect()
+{
+	// 200×100 source, landscape 900×480 → short side is the height.
+	// orient( 40, 5 ) landscape → 5 % of 480 = 24 height, width 24*(200/100)=48.
+	let img    = Image::new( solid_rgba( 200, 100 ), 200, 100 ).short_side( Length::orient( 40.0, 5.0 ) );
+	let canvas = Canvas::new( 900, 480 );
+	assert_eq!( img.preferred_size( 1000.0, &canvas ), ( 48.0, 24.0 ) );
+}
+
 // ── Arc sharing ───────────────────────────────────────────────────────────
 
 #[ test ]