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/entities/dimension.rs

@@ -1372,7 +1372,6 @@ fn tessellate_dimension_inner(
                     key_vertices: vec![],
                     aabb: WireModel::UNBOUNDED_AABB,
                     plinegen: true,
-                    vp_scissor: None,
                     fill_tris: vec![],
                     fill_tris_low: Vec::new(),
                 });
@@ -1399,7 +1398,6 @@ fn tessellate_dimension_inner(
                     key_vertices: vec![],
                     aabb: WireModel::UNBOUNDED_AABB,
                     plinegen: true,
-                    vp_scissor: None,
                     fill_tris: vec![],
                     fill_tris_low: Vec::new(),
                 });
@@ -1426,7 +1424,6 @@ fn tessellate_dimension_inner(
                 key_vertices: vec![],
                 aabb: WireModel::UNBOUNDED_AABB,
                 plinegen: true,
-                vp_scissor: None,
                 fill_tris: vec![],
                 fill_tris_low: Vec::new(),
             });
@@ -1454,7 +1451,6 @@ fn tessellate_dimension_inner(
         key_vertices,
         aabb: WireModel::UNBOUNDED_AABB,
         plinegen: true,
-        vp_scissor: None,
         fill_tris: geom.arrow_fill,
         // fill_tris_low intentionally empty: this fill renders on the top-level
         // path, where consumers (face3d_gpu, xclip) treat a short low half as
@@ -1498,7 +1494,6 @@ fn tessellate_dimension_inner(
                     key_vertices: vec![],
                     aabb: WireModel::UNBOUNDED_AABB,
                     plinegen: true,
-                    vp_scissor: None,
                     fill_tris: rect,
                     // fill_tris_low intentionally empty: this fill renders on the
                     // top-level path, where consumers (face3d_gpu, xclip) treat a

src/entities/leader.rs

@@ -610,7 +610,6 @@ impl LeaderTess for Leader {
                 key_vertices: vec![],
                 aabb: WireModel::UNBOUNDED_AABB,
                 plinegen: true,
-                vp_scissor: None,
                 fill_tris: vec![],
                 fill_tris_low: Vec::new(),
             };
@@ -717,7 +716,6 @@ impl LeaderTess for Leader {
             key_vertices,
             aabb: WireModel::UNBOUNDED_AABB,
             plinegen: true,
-            vp_scissor: None,
             fill_tris,
             fill_tris_low: Vec::new(),
         }

src/entities/multileader.rs

@@ -1424,7 +1424,6 @@ impl MultiLeaderTess for MultiLeader {
             key_vertices: key_verts,
             aabb: WireModel::UNBOUNDED_AABB,
             plinegen: true,
-            vp_scissor: None,
             fill_tris: arrow_fill,
             // fill_tris_low intentionally empty: this fill renders on the
             // top-level path, where consumers (face3d_gpu, xclip) treat a short
@@ -1717,7 +1716,6 @@ impl MultiLeaderTess for MultiLeader {
                         key_vertices: vec![],
                         aabb: [nx as f32, ny as f32, xx as f32, xy as f32],
                         plinegen: true,
-                        vp_scissor: None,
                         fill_tris: vec![],
                         fill_tris_low: Vec::new(),
                     });
@@ -1782,7 +1780,6 @@ impl MultiLeaderTess for MultiLeader {
                         key_vertices: vec![],
                         aabb: WireModel::UNBOUNDED_AABB,
                         plinegen: true,
-                        vp_scissor: None,
                         fill_tris,
                         fill_tris_low: Vec::new(),
                     });
@@ -1818,7 +1815,6 @@ impl MultiLeaderTess for MultiLeader {
                         key_vertices: vec![],
                         aabb: WireModel::UNBOUNDED_AABB,
                         plinegen: true,
-                        vp_scissor: None,
                         fill_tris: vec![],
                         fill_tris_low: Vec::new(),
                     });

src/entities/table.rs

@@ -773,7 +773,6 @@ pub fn tessellate_table(
                 key_vertices: vec![],
                 aabb: WireModel::UNBOUNDED_AABB,
                 plinegen: true,
-                vp_scissor: None,
                 fill_tris,
                 // fill_tris_low intentionally empty: this fill renders on the
                 // top-level path, where consumers (face3d_gpu, xclip) treat a