feat: GPU scissor rect for paper-space viewport wires

Viewport-projected wires now clip at pixel level to their viewport boundary.

- WireModel.vp_scissor: Option<[f32; 4]> — paper-space bbox set on projection
- WireGpu.vp_scissor carries it through to the draw loop
- Pipeline.compute_wire_scissors() converts paper→NDC→pixel each frame
- render() calls set_scissor_rect() per wire, reset after viewport group
- INTEGRATION_GAPS.md: Viewport ⚠️ → ✅, all 51 gaps resolved

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/scene/render.rs

@@ -131,6 +131,7 @@ impl shader::Primitive for Primitive {
             pipeline.upload_face3d(device, &self.face3d_wires[..]);
             pipeline.cached_epoch = self.geometry_epoch;
         }
+        pipeline.compute_wire_scissors(self.uniforms.view_proj, clip_size.width, clip_size.height);
         if self.show_viewcube {
             pipeline.viewcube.upload(
                 queue,