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/element.rs

@@ -66,8 +66,8 @@ impl<Msg: Clone> Element<Msg>
 			Element::VSlider( s )     => s.preferred_size( max_width, canvas ),
 			Element::Container( c )   => c.preferred_size( max_width, canvas ),
 			Element::Toggle( t )      => t.preferred_size( max_width, canvas ),
-			Element::Separator( s )   => s.preferred_size( max_width ),
-			Element::ProgressBar( p ) => p.preferred_size( max_width ),
+			Element::Separator( s )   => s.preferred_size( max_width, canvas ),
+			Element::ProgressBar( p ) => p.preferred_size( max_width, canvas ),
 			Element::Checkbox( c )    => c.preferred_size( max_width, canvas ),
 			Element::Radio( r )       => r.preferred_size( max_width, canvas ),
 			Element::ListItem( l )    => l.preferred_size( max_width, canvas ),
@@ -75,7 +75,7 @@ impl<Msg: Clone> Element<Msg>
 			Element::Pressable( p )   => p.preferred_size( max_width, canvas ),
 			Element::Flex( f )        => f.preferred_size( max_width, canvas ),
 			Element::AnchoredOverlay( a ) => a.child.preferred_size( max_width, canvas ),
-			Element::Spinner( s )         => s.preferred_size( max_width ),
+			Element::Spinner( s )         => s.preferred_size( max_width, canvas ),
 			Element::External( e )        => e.preferred_size( max_width ),
 			Element::Carousel( c )        => c.preferred_size( max_width, canvas ),
 		}
@@ -329,6 +329,9 @@ impl<Msg: Clone> Element<Msg>
 					on_change: s.on_change.clone(),
 					axis:      slider::SliderAxis::Horizontal,
 					value:     s.value,
+					// Design-px fallback; the layout pass overwrites this with
+					// the widget-scaling-resolved size via `set_slider_thumb_px`.
+					thumb_px:  slider::thumb_design_px(),
 				}
 			}
 			Element::VSlider( s ) =>
@@ -338,6 +341,9 @@ impl<Msg: Clone> Element<Msg>
 					on_change: s.on_change.clone(),
 					axis:      slider::SliderAxis::Vertical,
 					value:     s.value,
+					// The vertical axis maps against the full rect height and
+					// takes no thumb inset, so this is unused.
+					thumb_px:  0.0,
 				}
 			}
 			_ => WidgetHandlers::None,