refactor(scene/mod.rs): pull entity-specific helpers into entities/

Five chunks moved out of the 5965-line scene/mod.rs to live with the
entities they belong to:

- wide_lwpolyline_fills → entities/lwpolyline.rs::wide_fills
- wide_polyline2d_fills → entities/polyline.rs::wide_fills
- polyline_segment_fill → entities/common.rs (shared by both above)
- text_obb_corners_native + text_baseline_points + text_greek_obb_tris
  → entities/text_support.rs (handles Text/MText/Attribute alike)
- append_insert_attribute_wires → entities/insert.rs

Net: scene/mod.rs 5965 → 5404 lines (~560 lines extracted). Three
helpers in scene/render.rs / scene/block_cache.rs widened from
pub(super) to pub(crate) so the relocated functions can still reach
them. scene::entity_aabb also widened for the same reason.

scene::tessellate_entity_dim_text stays put — it's a thin wrapper
around the private tessellate_entity dispatcher and moving it would
require exposing tessellate_entity's full signature across the crate.

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

src/entities/lwpolyline.rs

@@ -516,3 +516,36 @@ impl Transformable for LwPolyline {
         apply_transform(self, t);
     }
 }
+pub(crate) fn wide_fills(pl: &acadrust::entities::LwPolyline) -> Vec<Vec<[f32; 2]>> {
+    let hw_const = (pl.constant_width / 2.0) as f32;
+    let verts = &pl.vertices;
+    let n = verts.len();
+    if n < 2 {
+        return vec![];
+    }
+    let seg_count = if pl.is_closed { n } else { n - 1 };
+    let mut out = Vec::new();
+    for i in 0..seg_count {
+        let v0 = &verts[i];
+        let v1 = &verts[(i + 1) % n];
+        let hw0 = if v0.start_width > 1e-9 {
+            v0.start_width as f32 / 2.0
+        } else {
+            hw_const
+        };
+        let hw1 = if v0.end_width > 1e-9 {
+            v0.end_width as f32 / 2.0
+        } else {
+            hw_const
+        };
+        if hw0 < 1e-6 && hw1 < 1e-6 {
+            continue;
+        }
+        let p0 = [v0.location.x as f32, v0.location.y as f32];
+        let p1 = [v1.location.x as f32, v1.location.y as f32];
+        if let Some(poly) = crate::entities::common::polyline_segment_fill(p0, p1, hw0, hw1, v0.bulge as f32) {
+            out.push(poly);
+        }
+    }
+    out
+}