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 <[email protected]>

src/scene/pipeline/hatch_web_gpu.rs

@@ -58,11 +58,9 @@ struct HatchWebUniform {
 pub struct HatchWebGpu {
     pub vertex_buffer: wgpu::Buffer,
     pub bind_group: wgpu::BindGroup,
-    /// Reserved for viewport-clip scissor / per-frame AABB LOD (mirrors
-    /// `WipeoutGpu`); not yet wired into the web hatch draw loop — the native
-    /// batched path doesn't do per-hatch scissor/LOD either.
-    #[allow(dead_code)]
-    pub vp_scissor: Option<[f32; 4]>,
+    /// Reserved for per-frame AABB LOD (mirrors `WipeoutGpu`); not yet wired
+    /// into the web hatch draw loop — the native batched path doesn't do
+    /// per-hatch LOD either.
     #[allow(dead_code)]
     pub world_aabb: [f32; 4],
     _uniform_buf: wgpu::Buffer,
@@ -344,7 +342,6 @@ impl HatchWebGpu {
         Self {
             vertex_buffer,
             bind_group,
-            vp_scissor: model.vp_scissor,
             world_aabb,
             _uniform_buf,
             _data_tex: data_tex,