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/image_gpu.rs

@@ -66,8 +66,6 @@ pub struct ImageGpu {
     /// more when the raster is clipped to a triangulated polygon.
     pub vertex_count: u32,
     pub bind_group: wgpu::BindGroup,
-    /// Mirrors `ImageModel.vp_scissor` — see `HatchGpu.vp_scissor`.
-    pub vp_scissor: Option<[f32; 4]>,
     _texture: wgpu::Texture,
     _sampler: wgpu::Sampler,
     _params_buf: wgpu::Buffer,
@@ -186,7 +184,6 @@ impl ImageGpu {
             vertex_buffer,
             vertex_count,
             bind_group,
-            vp_scissor: model.vp_scissor,
             _texture: texture,
             _sampler,
             _params_buf,