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 <[email protected]>

src/scene/render.rs

@@ -279,7 +279,7 @@ pub(super) fn render_style_for(
 /// Like `render_style_for` but resolves ByBlock properties by inheriting from
 /// the INSERT entity's already-resolved style. Call this for exploded block
 /// sub-entities so that ByBlock color/linetype/lineweight propagate correctly.
-pub(super) fn render_style_for_block_sub(
+pub(crate) fn render_style_for_block_sub(
     document: &CadDocument,
     e: &EntityType,
     insert_color: [f32; 4],
@@ -314,7 +314,7 @@ pub(super) fn render_style_for_block_sub(
 /// Adapt white→black or black→white based on background luminance.
 /// White entities on light backgrounds become black, black entities on dark
 /// backgrounds become white. All other colors pass through unchanged.
-pub(super) fn adapt_to_bg(color: [f32; 4], bg: [f32; 4]) -> [f32; 4] {
+pub(crate) fn adapt_to_bg(color: [f32; 4], bg: [f32; 4]) -> [f32; 4] {
     let lum = 0.299 * bg[0] + 0.587 * bg[1] + 0.114 * bg[2];
     let is_white = color[0] > 0.95 && color[1] > 0.95 && color[2] > 0.95;
     let is_black = color[0] < 0.05 && color[1] < 0.05 && color[2] < 0.05;