fix(renderer): keep draw-order offsets within clipping planes

src/shaders/draw_order.wgsl

@@ -0,0 +1,13 @@
+// Positive order draws in front; zero retains geometric depth. Keep the usual
+// 0.001 NDC offset away from the clipping planes, but reduce it near either
+// plane: perspective depth can be much closer to 1 than this fixed offset.
+//
+// Using order / (1 + abs(order)) for the limited region keeps ordering monotonic
+// (including the paper background's -2). Less than half the remaining depth is
+// consumed, so valid vertices stay valid. Out-of-range/behind-eye vertices get
+// no offset and remain subject to the real near/far clipping planes.
+fn apply_draw_order(clip: vec4<f32>, order: f32) -> vec4<f32> {
+    let room = 0.5 * max(0.0, min(clip.z, clip.w - clip.z));
+    let scale = min(0.001 * max(clip.w, 0.0), room / (1.0 + abs(order)));
+    return vec4<f32>(clip.xy, clip.z - order * scale, clip.w);
+}