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/entity.rs

@@ -727,7 +727,6 @@ impl Scene {
                     angle_offset: 0.0,
                     scale: 1.0,
                     world_origin: fill_origin,
-                    vp_scissor: None,
                     draw_depth: depth_map.get(&common.handle.value()).copied().unwrap_or(0.0),
                 });
             }
@@ -1002,7 +1001,6 @@ impl Scene {
                 angle_offset: 0.0,
                 scale: 1.0,
                 world_origin: origin,
-                vp_scissor: None,
                 draw_depth: 0.0,
             });
         }
@@ -1058,7 +1056,6 @@ impl Scene {
                     angle_offset: 0.0,
                     scale: 1.0,
                     world_origin: fill_origin,
-                    vp_scissor: None,
                     draw_depth: 0.0,
                 });
             }
@@ -1593,7 +1590,6 @@ impl Scene {
             angle_offset: if prebaked { 0.0 } else { dxf.pattern_angle as f32 },
             scale: if prebaked { 1.0 } else { dxf.pattern_scale as f32 },
             world_origin,
-            vp_scissor: None,
             draw_depth: 0.0,
         })
     }
@@ -1808,7 +1804,6 @@ impl Scene {
             angle_offset: 0.0,
             scale: 1.0,
             world_origin,
-            vp_scissor: None,
             draw_depth: 0.0,
         }
     }