perf(scene): extend viewport scissor to hatch / image / wipeout passes (Phase 1.4)

Wire pass already uses `WireModel.vp_scissor` to clip viewport-projected
geometry to its paper-space viewport rect; hatch / image / wipeout
passes drew across the whole canvas. When a future paper-space path
projects fills/images into a viewport, they would bleed past the
viewport frame.

Add `vp_scissor: Option<[f32; 4]>` to HatchModel and ImageModel
(mirroring WireModel), forward it through HatchGpu / ImageGpu, and add
per-pass `compute_*_scissors` / `set_scissor_rect` machinery. Factored
the wire-side scissor-projection math into a shared `project_scissor`
helper. Field defaults to None at every constructor site — no producer
sets it yet, so this is plumbing for the paper-viewport fill projection
that Phase 1.4 unblocks without changing current visual output.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/scene/mod.rs

@@ -2315,6 +2315,7 @@ impl Scene {
                     color,
                     angle_offset: 0.0,
                     scale: 1.0,
+                    vp_scissor: None,
                 });
             }
         }
@@ -2367,6 +2368,7 @@ impl Scene {
                     color: fill_color,
                     angle_offset: 0.0,
                     scale: 1.0,
+                    vp_scissor: None,
                 });
             }
         }
@@ -2658,6 +2660,7 @@ impl Scene {
             color,
             angle_offset: dxf.pattern_angle as f32,
             scale: dxf.pattern_scale as f32,
+            vp_scissor: None,
         })
     }
 
@@ -2806,6 +2809,7 @@ impl Scene {
             color,
             angle_offset: 0.0,
             scale: 1.0,
+            vp_scissor: None,
         }
     }