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/input/gesture/mod.rs

@@ -47,6 +47,17 @@ mod tests;
 
 // ─── State ───────────────────────────────────────────────────────────────────
 
+/// Axis a swipe locked onto during its first 8 px of travel. Once set,
+/// the perpendicular axis is ignored for the rest of the gesture so a
+/// vertical swipe that drifts sideways never also drives the pager (and
+/// vice-versa).
+#[ derive( Debug, Clone, Copy, PartialEq, Eq ) ]
+pub enum SwipeAxis
+{
+	Vertical,
+	Horizontal,
+}
+
 /// Per-surface gesture tracking. Lives on `SurfaceState::gesture`.
 ///
 /// Every field is `pub` because `AppData`'s long-press deadline
@@ -81,6 +92,10 @@ pub struct GestureState<Msg: Clone>
 	/// Vertical drag exceeded the 8 px threshold. Release runs the
 	/// swipe-up / swipe-down commit check instead of the button path.
 	pub vertical_drag_started:   bool,
+	/// Axis this swipe locked onto on its first 8 px of travel. Pins the
+	/// gesture to one axis so vertical and horizontal swipes stay
+	/// mutually exclusive. `None` until the lock fires.
+	pub swipe_axis:              Option<SwipeAxis>,
 	/// Slider widget being dragged for a live-value update.
 	pub dragging_slider:         Option<usize>,
 	/// Instant when the long-press window opened. `None` if the hit
@@ -145,6 +160,7 @@ impl<Msg: Clone> GestureState<Msg>
 			scroll_drag_started:     false,
 			horizontal_drag_started: false,
 			vertical_drag_started:   false,
+			swipe_axis:              None,
 			dragging_slider:         None,
 			long_press_start:        None,
 			long_press_origin:       None,
@@ -190,6 +206,7 @@ impl<Msg: Clone> GestureState<Msg>
 		self.scroll_drag_started     = false;
 		self.horizontal_drag_started = false;
 		self.vertical_drag_started   = false;
+		self.swipe_axis              = None;
 		self.pressed_idx             = hit;
 		// A fresh press resets any stale long-press / source state
 		// from a prior gesture that never released cleanly. The
@@ -335,22 +352,33 @@ impl<Msg: Clone> GestureState<Msg>
 			return MoveOutcome::Idle;
 		}
 
-		// Swipe progress — independent on both axes so a vertical-
-		// dominant gesture that picks up horizontal drift still drives
-		// the pager.
+		// Swipe progress — locked to a single axis (see below) so a
+		// gesture only ever drives one of the vertical panels or the
+		// horizontal pager, never both.
 		if let Some( start ) = self.start
 		{
 			let dy = start.y - pos.y;
 			let dx = pos.x - start.x;
 
-			let ( up, down ) = if swipe.surface_height > 0
+			// Lock onto the dominant axis on the first 8 px of travel so
+			// vertical and horizontal swipes stay mutually exclusive.
+			if self.swipe_axis.is_none() && dx.abs().max( dy.abs() ) > 8.0
+			{
+				self.swipe_axis = Some( if dy.abs() >= dx.abs() { SwipeAxis::Vertical } else { SwipeAxis::Horizontal } );
+			}
+			let vertical_allowed   = self.swipe_axis != Some( SwipeAxis::Horizontal );
+			let horizontal_allowed = self.swipe_axis != Some( SwipeAxis::Vertical );
+
+			let ( up, down ) = if vertical_allowed && swipe.surface_height > 0
 			{
 				let h = swipe.surface_height as f32;
 				if dy > 0.0
 				{
 					let threshold = h * swipe.up_thresh;
 					if dy.abs() > 8.0 { self.vertical_drag_started = true; }
-					( Some( ( dy / threshold ).clamp( 0.0, 1.0 ) ), None )
+					// Unclamped on purpose — lets follow-the-finger
+					// panels track past the commit threshold.
+					( Some( ( dy / threshold ).max( 0.0 ) ), None )
 				}
 				else if start.y <= h * swipe.down_edge
 				{
@@ -364,7 +392,7 @@ impl<Msg: Clone> GestureState<Msg>
 			}
 			else { ( None, None ) };
 
-			let horizontal = if swipe.surface_width > 0
+			let horizontal = if horizontal_allowed && swipe.surface_width > 0
 			{
 				let h_thr      = swipe.surface_width as f32 * swipe.horizontal_thresh;
 				let h_progress = if h_thr > 0.0 { dx / h_thr } else { 0.0 };