Restore abs() in the rect shader, and validate every shader without a device

`7c50a3e` renamed a length's `abs` component to `px` and took the WGSL `abs()`
builtin in the rounded-rect distance with it, so every window failed shader
validation on the first frame while `cargo test` stayed green. `naga` is
reachable through `wgpu`, so a unit test now composes each shader file with the
prelude the way the renderer does and parses and validates it; it reads the
shader directory rather than naming primitives, so a new one is covered by
adding its file.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

core/src/render/shader/rect.wgsl

@@ -34,6 +34,6 @@ fn distance_from_rect(pixel_pos: vec2<f32>, rect_center: vec2<f32>, rect_corner:
     // vec from center to pixel
     let p = pixel_pos - rect_center;
     // vec from inner rect corner to pixel
-    let q = px(p) - (rect_corner - radius);
+    let q = abs(p) - (rect_corner - radius);
     return length(max(q, vec2(0.0))) - radius;
 }