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/modules/annotate/leader_cmd.rs

@@ -203,6 +203,7 @@ fn preview_wire(pts: &[Vec3]) -> WireModel {
             key_vertices: vec![],
             aabb: WireModel::UNBOUNDED_AABB,
             plinegen: true,
+            vp_scissor: None,
     }
 }