Merge branch 'worktree-agent-a22a049c461818dbe' into integration

# Conflicts:
#	core/dr-pipeline/tests/mask_sidecar.rs

core/dr-gpu/src/shaders/mask.wgsl

@@ -43,7 +43,9 @@ struct MaskParams {
     // `falloff_code` on the Rust side.
     falloff: u32,
 
-    // Geometry, in normalised output coordinates. Meaning depends on `mode`.
+    // Geometry. Meaning depends on `mode`. The centre is in normalised 0..1
+    // coordinates; every distance below it is in the isotropic frame units
+    // `frame_delta` establishes.
     centre: vec2<f32>,
     // Linear: (cos, sin) of the ramp direction. Radial: semi-axes.
     axis: vec2<f32>,
@@ -128,9 +130,23 @@ fn region_mask(px: vec2<i32>) -> f32 {
     return total / n;
 }
 
+// Offset from a gradient's centre, in the frame's own **isotropic** units:
+// y spans 0..1 and x spans 0..aspect, so a step of the same length means the
+// same distance whichever way it points.
+//
+// Without this the geometry lives in raw 0..1, where one axis is compressed
+// against the other by the aspect ratio — so a 45° ramp is not at 45° on
+// anything but a square frame, and a radial with equal radii draws an ellipse.
+// Both faults are invisible in the stored numbers and obvious the moment a
+// handle is dragged on a photograph, which is what this exists for.
+fn frame_delta(uv: vec2<f32>) -> vec2<f32> {
+    let aspect = vec2<f32>(f32(p.width) / f32(max(p.height, 1u)), 1.0);
+    return (uv - p.centre) * aspect;
+}
+
 fn linear_mask(uv: vec2<f32>) -> f32 {
     // Signed distance along the ramp direction, from the centre.
-    let d = dot(uv - p.centre, p.axis);
+    let d = dot(frame_delta(uv), p.axis);
     if (p.softness <= 0.0) {
         return select(0.0, 1.0, d >= 0.0);
     }
@@ -140,7 +156,7 @@ fn linear_mask(uv: vec2<f32>) -> f32 {
 fn radial_mask(uv: vec2<f32>) -> f32 {
     let ca = cos(-p.angle);
     let sa = sin(-p.angle);
-    let d = uv - p.centre;
+    let d = frame_delta(uv);
     // Into the ellipse's own frame, then normalised by its semi-axes so the
     // problem becomes a unit circle.
     let local = vec2<f32>(d.x * ca - d.y * sa, d.x * sa + d.y * ca);