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/event_loop/app_data.rs

@@ -447,6 +447,7 @@ impl<A: App> AppData<A>
 		// current buffer scale before handing the dimensions to the app.
 		let sf = self.main.scale_factor.max( 1 ) as u32;
 		crate::types::set_viewport_size( w * sf, h * sf );
+		crate::types::set_density( sf as f32 );
 		self.app.on_scale_changed( sf );
 		self.app.on_resize( w * sf, h * sf );
 		// `on_resize` may flip app-state that the view depends on (apps that