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/view/render.rs

@@ -387,10 +387,7 @@ impl shader::Primitive for Primitive {
                 #[cfg(not(target_arch = "wasm32"))]
                 let mut _patched = false;
                 #[cfg(not(target_arch = "wasm32"))]
-                if crate::scene::wire_gpu_patch_enabled()
-                    && inner.wire_const_bgl.is_some()
-                    && crate::scene::pipeline::wire_arena::is_arena_eligible(&vp.wires)
-                {
+                if crate::scene::wire_gpu_patch_enabled() && inner.wire_const_bgl.is_some() {
                     use crate::scene::pipeline::wire_arena::{self, WireArena};
                     // Split the resident set into the regular 2D wires and the
                     // mesh/solid EDGE wires (which need the mesh-edge draw
@@ -576,9 +573,6 @@ impl shader::Primitive for Primitive {
                 inner.upload_silhouettes(device, &[], vp.view_dir);
             }
             inner.upload_clip_boundary(device, &vp.clip_boundary_ndc);
-            inner.compute_wire_scissors(view_rot, eye, clip_size.width, clip_size.height);
-            inner.compute_wipeout_scissors(view_rot, eye, clip_size.width, clip_size.height);
-            inner.compute_image_scissors(view_rot, eye, clip_size.width, clip_size.height);
             inner.compute_hatch_lod(queue, view_rot, eye, clip_size.width, clip_size.height);
             inner.compute_wipeout_lod(view_rot, eye, clip_size.width, clip_size.height);
             inner.compute_mesh_lod(view_rot, eye, clip_size.width, clip_size.height);