ltk: subsurface slides over overlays, axis-locked swipes, physical-space layout, touch reset on resume

Subsurfaces can now be parented to an overlay surface, not just the main surface. `SubsurfaceSpec` gains `parent: SubsurfaceParent { Main, Overlay(id) }` so a sliding panel can ride above app windows the way an overlay panel does, and an optional `gpu: bool` so content that uses the `surface-panel` backdrop-filter glass (a GLES-only pass) keeps it while sliding instead of dropping to the software rasteriser. `reconcile_subsurfaces` resolves each spec's parent independently — skipping an `Overlay` parent that is absent, unconfigured, or zero-sized — tracks the rastered size on the slot, and commits each touched parent once per frame. The ~14 GLES shader programs are compiled once into a shared `AppData::subsurface_gles_canvas` reused across every subsurface, so a lazily re-created sliding panel never recompiles them (hundreds of ms on a mobile GPU).
Vertical and horizontal swipes are now mutually exclusive: a gesture locks onto its dominant axis within the first 8 px of travel (new `SwipeAxis`) and ignores the perpendicular axis for the rest of the gesture, so a vertical swipe that drifts sideways no longer also drives the pager (and vice-versa). The upward swipe progress is no longer clamped at 1.0 — follow-the-finger panels can keep tracking the finger past the commit threshold — and a release below threshold delivers a final `progress = 0.0` cancellation pulse. A vertical swipe also no longer re-rasters the full-screen main surface on every motion event: only the overlays it drives are refreshed, while the horizontal pager (which does move the main surface) still redraws it. Re-rastering the main on every frame of a vertical drag was wasted work that stalled the loop and made the gesture feel laggy to start on a slow GPU.
Layout-affecting `Length` values (widths, paddings, gaps, widget sizes, including `Vw` / `Vh`) now resolve against the physical viewport via the new `Canvas::viewport_layout()` — the space the layout tree is actually computed in — so a `Vw(100)` fills the surface on a HiDPI (scale 2) output instead of covering half of it. Font sizes are unchanged: they still resolve against the logical viewport and are scaled at raster time. Switched column, row, spacer, wrap_grid, container, image, and vslider over to it; `img_widget` and `vslider` now document `Length::vw` / `vh` sizing and the showcase example demonstrates a viewport-relative image.
Touch gesture state is reset when the touch capability is added or removed — suspend / resume on devices that power the touchscreen down. A yanked capability never delivers the pending `up` / `cancel`, so the shared `reset_touch_state()` (also used by the `wl_touch.cancel` handler) drops the stranded `primary_touch_id` / slot state across the main surface and every overlay, keeping the first post-resume gesture clean. Also drops an accidental duplicate `on_scale_changed` call from the scale-change handler.

src/render/mod.rs

@@ -224,6 +224,19 @@ impl Canvas
 		}
 	}
 
+	/// `(width, height)` of the surface in **physical** pixels — the same
+	/// space the layout tree is computed in (the root rect is `pw × ph`).
+	/// This is the viewport that layout-affecting [`crate::Length`] values
+	/// (widths, paddings, gaps, widget sizes) must resolve against so a
+	/// `Vw(100)` fills the surface. Text font sizes are the exception:
+	/// they resolve against [`Self::viewport_logical`] and are then scaled
+	/// by `dpi_scale` at raster time, so they must NOT use this.
+	pub fn viewport_layout( &self ) -> ( f32, f32 )
+	{
+		let ( pw, ph ) = self.size();
+		( pw as f32, ph as f32 )
+	}
+
 	/// Borrow the GLES texture backing this canvas, when the canvas
 	/// is GPU-backed.
 	pub fn borrowed_gles_texture( &self ) -> Option<BorrowedGlesTexture>
@@ -697,4 +710,16 @@ mod viewport_tests
 		c.set_dpi_scale( 0.0 );
 		assert_eq!( c.viewport_logical(), ( 800.0, 600.0 ) );
 	}
+
+	#[ test ]
+	fn viewport_layout_is_physical_and_ignores_dpi_scale()
+	{
+		// Layout-affecting `Length` values resolve against this, so it must
+		// stay in physical space (where the layout tree is computed) even on
+		// HiDPI — unlike `viewport_logical`, it does not divide by the scale.
+		let mut c = Canvas::new( 720, 1440 );
+		assert_eq!( c.viewport_layout(), ( 720.0, 1440.0 ) );
+		c.set_dpi_scale( 2.0 );
+		assert_eq!( c.viewport_layout(), ( 720.0, 1440.0 ) );
+	}
 }