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/pipeline/wipeout_gpu.rs

@@ -119,10 +119,6 @@ pub struct FamilyBatchData {
 pub struct WipeoutGpu {
     pub vertex_buffer: wgpu::Buffer,
     pub bind_group: wgpu::BindGroup,
-    /// Mirrors `HatchModel.vp_scissor`. Forwarded to the pipeline so each
-    /// frame's `compute_hatch_scissors` can translate the world rect into
-    /// a pixel scissor.
-    pub vp_scissor: Option<[f32; 4]>,
     /// World-space XY bounding rect [min_x, min_y, max_x, max_y] of the
     /// boundary polygon. Used by the per-frame LOD pass to skip hatches
     /// whose entire footprint projects to less than ~2 px (Phase 3.3).
@@ -382,7 +378,6 @@ impl WipeoutGpu {
         Self {
             vertex_buffer,
             bind_group,
-            vp_scissor: model.vp_scissor,
             world_aabb,
             _uniform_buf,
             _boundary_buf,