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 <noreply@anthropic.com>

src/scene/hatch_model.rs

@@ -58,4 +58,10 @@ pub struct HatchModel {
     pub angle_offset: f32,
     /// Pattern scale multiplier (from DXF `pattern_scale`).
     pub scale: f32,
+    /// Optional world-space XY rect [x0, y0, x1, y1] for paper-space
+    /// viewport clipping. When `Some`, the pipeline translates it into
+    /// a per-frame pixel scissor and clips this hatch's draw call to it,
+    /// preventing viewport-projected content from bleeding past the
+    /// viewport frame. Mirrors `WireModel.vp_scissor`.
+    pub vp_scissor: Option<[f32; 4]>,
 }