refactor(viewport): unify content-viewport clipping on the stencil path

Rectangular paper viewports now go through the same GPU stencil clip as
non-rectangular ones: viewport_clip_boundary_ndc returns the four rect
corners (a full-target mask) instead of nothing, so the render path has
no rect/non-rect special case — every content viewport carries a clip
boundary and draws with stencil reference 0xFF.

With a single clip path, the old per-wire / -image / -wipeout scissor
mechanism is fully dead and removed: compute_*_scissors, the
*_pixel_scissors buffers, the project_scissor helper, the per-pass
set_scissor_rect blocks, the vp_scissor field threaded through
WireModel / ImageModel / HatchModel and their GPU mirrors (WireGpu /
ImageGpu / WipeoutGpu / HatchWebGpu), from_run's scissor parameter, the
scissor grouping in build_wire_buffers, and the is_arena_eligible gate.
Net -245 lines across 54 files.

Rectangular viewports still clip exactly via their own render rectangle;
the stencil mask is a redundant full-target no-op there, kept only so
the clip code path stays uniform. XCLIP keeps its CPU wire clip.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/scene/model/image_model.rs

@@ -100,10 +100,6 @@ pub struct ImageModel {
     /// Low residual paired with `corners` (double-single) so the GPU keeps
     /// sub-unit precision at UTM-scale insertion points.
     pub corners_low: [[f32; 3]; 4],
-    /// Optional world-space XY rect [x0, y0, x1, y1] for paper-space
-    /// viewport clipping. Mirrors `WireModel.vp_scissor` /
-    /// `HatchModel.vp_scissor`.
-    pub vp_scissor: Option<[f32; 4]>,
     /// Normalized draw-order depth in (0,1); higher draws on top. Fed to the
     /// image pipeline as a small clip-z bias so the raster orders correctly
     /// against other entity types.
@@ -179,7 +175,6 @@ impl ImageModel {
             opacity,
             corners,
             corners_low,
-            vp_scissor: None,
             draw_depth: 0.0,
             verts,
         })
@@ -234,7 +229,6 @@ impl ImageModel {
             opacity: 1.0,
             corners,
             corners_low,
-            vp_scissor: None,
             draw_depth: 0.0,
             verts,
         })