Resolve fluid geometry bounds in logical space and let the runtime set the density from the buffer scale

Stock widgets size themselves with `Length::fluid`, whose auto-clamp `[0.7·px, 1.5·px]` is documented as logical px, yet `Canvas::resolve_geom` resolved it against the physical viewport and applied the bounds to the physical value. On any HiDPI surface whose physical short side exceeds 618 px (412 × 1.5) the cap engaged: a 1080 × 1920 phone at scale 2 got its geometry at 1.5 physical px per design px, that is 0.75 logical, while fonts, resolved by `font_px` against the logical viewport and scaled at raster time, kept growing to 2.62. Rows, paddings and icons came out at 60 % of their intended size with the text overflowing them; at scale 1 physical and logical coincide, so a winit session never showed it.
`Length::resolve_physical` takes the buffer scale and multiplies `min_px` / `max_px` by it before clamping; `resolve_with_density` delegates to it with scale 1. `Canvas::resolve_geom` passes its `dpi_scale`, so every stock-widget geometry shares the surface factor of the fonts, and `overlays_reconcile` does the same for `OverlaySpec::size`, which had the same mix. A zero scale falls back to unscaled bounds.
The event loop now calls `set_density( sf )` right before `App::on_scale_changed`, both on `configure` and on `scale_factor_changed` of the main surface, so `Length::dp( n )` means n logical px in every app without plumbing; an app wanting a DPI-derived density still overrides it from that callback, and crustace already sets the same value there. Two tests cover the clamp at scale 2 and the geometry/font parity on a 1080 × 2460 canvas.

src/render/mod.rs

@@ -318,7 +318,7 @@ impl Canvas
 	/// a `Length::dp` override follows the canvas the widget draws on.
 	pub fn resolve_geom( &self, l: Length ) -> f32
 	{
-		l.resolve_with_density( self.viewport_layout(), Length::EM_BASE_DEFAULT, self.density() )
+		l.resolve_physical( self.viewport_layout(), Length::EM_BASE_DEFAULT, self.density(), self.dpi_scale() )
 	}
 
 	/// Resolve an explicit [`Length`] in **font** space: against
@@ -960,6 +960,23 @@ mod viewport_tests
 		set_widget_scaling( WidgetScaling::Fluid );
 	}
 
+	#[ test ]
+	fn geom_px_fluid_cap_is_logical_on_hidpi()
+	{
+		use crate::types::{ set_widget_scaling, WidgetScaling };
+		let _g = crate::TEST_GLOBALS_LOCK.lock().unwrap_or_else( |e| e.into_inner() );
+		set_widget_scaling( WidgetScaling::Fluid );
+
+		// 1080 × 2460 phone at buffer scale 2: geometry and fonts must share
+		// the same surface factor (2.62 physical px per design px).
+		let mut c = Canvas::new( 1080, 2460 );
+		c.set_dpi_scale( 2.0 );
+		let geom = c.geom_px( 48.0 );
+		let font = c.font_px( 48.0 ) * 2.0;
+		assert!( ( geom - 48.0 / 412.0 * 1080.0 ).abs() < 1e-2 );
+		assert!( ( geom - font ).abs() < 1e-2 );
+	}
+
 	#[ test ]
 	fn canvas_density_overrides_process_density()
 	{