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/wire_model.rs

@@ -63,6 +63,9 @@ pub struct WireModel {
     /// When false the linetype pattern restarts at each NaN-separated segment
     /// (DXF PLINEGEN=0).  When true the pattern runs continuously (PLINEGEN=1).
     pub plinegen: bool,
+    /// Paper-space bounding box [x0, y0, x1, y1] for GPU scissor clipping.
+    /// Set only for viewport-projected wires in paper-space layouts.
+    pub vp_scissor: Option<[f32; 4]>,
 }
 
 impl WireModel {
@@ -89,6 +92,7 @@ impl WireModel {
             key_vertices: vec![],
             aabb: Self::UNBOUNDED_AABB,
             plinegen: true,
+            vp_scissor: None,
         }
     }
 
@@ -189,6 +193,7 @@ impl Default for WireModel {
             key_vertices: Vec::new(),
             aabb: Self::UNBOUNDED_AABB,
             plinegen: true,
+            vp_scissor: None,
         }
     }
 }