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/radio/mod.rs

@@ -40,14 +40,14 @@ pub struct Radio<Msg: Clone>
 {
 	/// Whether this option is currently selected. Drawn from this field
 	/// every frame.
-	pub selected:  bool,
+	pub( crate ) selected:  bool,
 	/// Message emitted when the user picks this option. `None` leaves the
 	/// radio inert.
-	pub on_select: Option<Msg>,
+	pub( crate ) on_select: Option<Msg>,
 	/// Optional label drawn to the right of the circle.
-	pub label:     Option<String>,
+	pub( crate ) label:     Option<String>,
 	/// Optional stable identifier for focus management.
-	pub id:        Option<WidgetId>,
+	pub( crate ) id:        Option<WidgetId>,
 }
 
 impl<Msg: Clone> Radio<Msg>
@@ -83,14 +83,15 @@ impl<Msg: Clone> Radio<Msg>
 
 	pub fn preferred_size( &self, max_width: f32, canvas: &Canvas ) -> (f32, f32)
 	{
+		let outer = canvas.geom_px( theme::OUTER_SIZE );
 		let w = if let Some( ref label ) = self.label
 		{
-			let text_w = canvas.measure_text( label, theme::FONT_SIZE );
-			( theme::OUTER_SIZE + theme::GAP + text_w ).min( max_width )
+			let text_w = canvas.measure_text( label, canvas.font_px( theme::FONT_SIZE ) );
+			( outer + canvas.geom_px( theme::GAP ) + text_w ).min( max_width )
 		} else {
-			theme::OUTER_SIZE.min( max_width )
+			outer.min( max_width )
 		};
-		( w, theme::HEIGHT )
+		( w, canvas.geom_px( theme::HEIGHT ) )
 	}
 
 	/// Focus ring on the outer circle extends `FOCUS_W + 2 + FOCUS_W/2 ≈ 6.5 px`
@@ -102,28 +103,30 @@ impl<Msg: Clone> Radio<Msg>
 
 	pub fn draw( &self, canvas: &mut Canvas, rect: Rect, focused: bool )
 	{
-		let circle_y = rect.y + ( rect.height - theme::OUTER_SIZE ) / 2.0;
-		let outer_r  = theme::OUTER_SIZE / 2.0;
+		let outer_size = canvas.geom_px( theme::OUTER_SIZE );
+		let circle_y = rect.y + ( rect.height - outer_size ) / 2.0;
+		let outer_r  = outer_size / 2.0;
 		let outer_rect = Rect
 		{
 			x:      rect.x,
 			y:      circle_y,
-			width:  theme::OUTER_SIZE,
-			height: theme::OUTER_SIZE,
+			width:  outer_size,
+			height: outer_size,
 		};
 
 		if self.selected
 		{
 			canvas.fill_rect( outer_rect, theme::selected(), outer_r );
-			let dot_r = theme::DOT_SIZE / 2.0;
+			let dot_size = canvas.geom_px( theme::DOT_SIZE );
+			let dot_r = dot_size / 2.0;
 			let cx    = rect.x + outer_r;
 			let cy    = circle_y + outer_r;
 			let dot_rect = Rect
 			{
 				x:      cx - dot_r,
 				y:      cy - dot_r,
-				width:  theme::DOT_SIZE,
-				height: theme::DOT_SIZE,
+				width:  dot_size,
+				height: dot_size,
 			};
 			canvas.fill_rect( dot_rect, theme::dot_color(), dot_r );
 		} else {
@@ -138,9 +141,10 @@ impl<Msg: Clone> Radio<Msg>
 
 		if let Some( ref label ) = self.label
 		{
-			let text_x = rect.x + theme::OUTER_SIZE + theme::GAP;
-			let text_y = rect.y + ( rect.height + theme::FONT_SIZE ) / 2.0 - 2.0;
-			canvas.draw_text( label, text_x, text_y, theme::FONT_SIZE, theme::label_color() );
+			let fs     = canvas.font_px( theme::FONT_SIZE );
+			let text_x = rect.x + outer_size + canvas.geom_px( theme::GAP );
+			let text_y = rect.y + ( rect.height + fs ) / 2.0 - 2.0;
+			canvas.draw_text( label, text_x, text_y, fs, theme::label_color() );
 		}
 	}