Feat: Phase 5 — MLine, Tolerance render; DimStyle Choice picker on dimensions

- mline.rs: MLine renders center spine + two parallel offset lines at
  ±scale/2 using the vertex miter directions; vertex grips; scale,
  closed and style-name properties
- tolerance.rs: Tolerance renders the GDT text string via font
  tessellation at insertion_point with rotation from direction vector;
  insertion grip and position properties
- properties.rs: Dimension entity's style_name EditText is upgraded to
  a Choice dropdown when document.dim_styles is non-empty, enabling
  point-and-click DimStyle assignment without a separate panel

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/entities/mod.rs

@@ -10,6 +10,7 @@ pub mod leader;
 pub mod line;
 pub mod multileader;
 pub mod lwpolyline;
+pub mod mline;
 pub mod mtext;
 pub mod point;
 pub mod polyline;
@@ -18,5 +19,6 @@ pub mod ray;
 pub mod spline;
 pub mod text;
 mod text_support;
+pub mod tolerance;
 pub mod traits;
 pub mod viewport;