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/entities/traits.rs

@@ -3,11 +3,20 @@ use acadrust::{CadDocument, EntityType};
 use crate::command::EntityTransform;
 use crate::scene::acad_to_truck::TruckEntity;
 use crate::scene::object::{GripApply, GripDef, PropSection};
+use crate::scene::tess_util::LegacyGeometry;
 
 pub trait TruckConvertible {
     fn to_truck(&self, document: &CadDocument) -> Option<TruckEntity>;
 }
 
+/// Legacy fallback geometry for entities not covered by the truck
+/// topology pipeline (Viewport, Insert, Hatch outline, Ole2Frame, …).
+/// Returns world-offset-relative `f32` points + snap/key vertices the
+/// dispatcher wraps into a `WireModel`.
+pub trait LegacyTess {
+    fn legacy_geometry(&self, world_offset: [f64; 3]) -> LegacyGeometry;
+}
+
 pub trait Grippable {
     fn grips(&self) -> Vec<GripDef>;
     fn apply_grip(&mut self, grip_id: usize, apply: GripApply);