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/wire_model.rs

@@ -86,9 +86,6 @@ pub struct WireModel {
     /// share one interior phase (perpendicular dashes line up) yet each still
     /// ends on a dash. `None` (the default) = per-wire A-type / from-start.
     pub dash_align_end: Option<f32>,
-    /// 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]>,
     /// Pre-triangulated solid fill: flat vertex list, 3 per triangle (world-offset applied).
     /// Non-empty only for PolyfaceMesh / PolygonMesh entities.
     pub fill_tris: Vec<[f32; 3]>,
@@ -196,7 +193,6 @@ impl WireModel {
             plinegen: true,
             dash_from_start: false,
             dash_align_end: None,
-            vp_scissor: None,
             fill_tris: vec![],
             fill_tris_low: Vec::new(),
         }
@@ -406,7 +402,6 @@ impl Default for WireModel {
             plinegen: true,
             dash_from_start: false,
             dash_align_end: None,
-            vp_scissor: None,
             fill_tris: Vec::new(),
             fill_tris_low: Vec::new(),
             fill_is_3d: false,