refactor(tess): move per-entity tessellation into entities/

Each entity's tessellation now lives alongside its definition in
entities/*.rs, attached to the entity type via a local trait:

  LegacyTess     — Viewport / Insert / Hatch / Ole2Frame
                   (fallback path for entities not on the truck pipeline)
  LeaderTess     — Leader
  MultiLeaderTess — MultiLeader
  DimensionTess  — Dimension (carries its dim-only helpers too:
                   ~1700 lines of dim_geometry, dim_text_*, format_*,
                   tolerance, suppression, etc.)

scene/tessellate.rs keeps only the cross-entity dispatcher plus the
shared dim machinery reused by leader/multileader/dimension
(ArrowKind, DimGeom, append_arrow, arrow_from_block, add_segment,
add_polyline, normalized_or, push_tri, color_or_inherit, entity_z,
offset_snap_pts). Cross-entity arc helpers + aci_to_rgba moved to
scene/tess_util.rs.

scene/tessellate.rs shrinks from ~3300 to 809 lines. Compile clean,
all 9 tests pass.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/scene/render.rs

@@ -13,7 +13,7 @@ use std::sync::Arc;
 
 use super::pipeline::viewcube::{hover_id, VIEWCUBE_PX};
 use super::pipeline::Pipeline;
-use super::tessellate;
+use super::tess_util;
 use super::{HatchModel, ImageModel, MeshLodSet, Scene, Uniforms, WireModel};
 
 // ── PaperViewportPipeline / PaperViewportPrimitive ────────────────────────
@@ -242,7 +242,7 @@ pub(super) fn render_style_for(
             AcadColor::Index(i) => *i,
             _ => 0,
         };
-        let [r, g, b, _] = tessellate::aci_to_rgba(resolved);
+        let [r, g, b, _] = tess_util::aci_to_rgba(resolved);
         let alpha = 1.0 - e.common().transparency.as_percent() as f32;
         ([r, g, b, alpha], aci)
     };
@@ -473,7 +473,7 @@ impl Scene {
 
 // ── Linetype pattern helper ───────────────────────────────────────────────
 
-pub(super) fn resolve_pattern(
+pub(crate) fn resolve_pattern(
     table: &acadrust::tables::Table<LineType>,
     name: &str,
     scale: f32,