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/mod.rs

@@ -886,9 +886,7 @@ pub struct Scene {
     /// Cached wipeout fill models, keyed by geometry_epoch. Same
     /// reasoning as `hatch_cache`.
     wipeout_cache: RefCell<Option<(u64, Arc<Vec<HatchModel>>)>>,
-    /// Cached image models, keyed by geometry_epoch. Images do their own
-    /// per-frame culling in the GPU pipeline (vp_scissor); no camera key
-    /// needed here.
+    /// Cached image models, keyed by geometry_epoch. No camera key needed here.
     image_cache: RefCell<Option<(u64, Arc<Vec<ImageModel>>)>>,
     /// Cached mesh models, keyed by geometry_epoch.
     mesh_cache: RefCell<Option<(u64, Arc<Vec<MeshLodSet>>)>>,
@@ -2025,7 +2023,6 @@ impl Scene {
             color: self.paper_bg_color,
             angle_offset: 0.0,
             scale: 1.0,
-            vp_scissor: None,
             // Draw-order bias is signed: entity fills/wires land in (-1, 1)
             // (0 = neutral). A value below -1 forces the sheet strictly behind
             // EVERY object, in every case, with a tiny z offset (BIAS = 0.001,