fix(io): keep zero-radius circles and arcs on open

Problem

The open guard rejects circles and arcs with a radius below 1e-10. A normal
open fails when a drawing contains one. The user gets the recovery prompt.
AutoCAD and nanoCAD open these drawings.

The check came from ace99541. Its comment says the tessellator cannot sample
a degenerate curve. That was true of truck's parameter_division. Commit
8c7ebdc9 removed truck. Commit 96be8214 made the kernel sample circles and
arcs by sweep angle with a floored segment count. The reason for the check
no longer exists.

Change

- Remove the near-zero radius check for circles and arcs.
- Keep the checks for non-finite values and radii above 1e10.
- Update the guard comments. The purge rejects parser desync. It does not
  reject degenerate geometry. Be generous in what you accept and strict in
  what you produce.
- Add tests: a zero radius passes the guard, an absurd radius does not.
- Change the automation purge test sample from radius 0 to radius 1e11.

Behavior

- Zero-radius circles and arcs load. They render as dots.
- Tessellation, hit testing, snapping, and hatch boundaries accept a zero
  radius.
- The DXF writer does not change.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

src/app/automation.rs

@@ -1414,7 +1414,9 @@ mod tests {
         doc.add_entity(acadrust::EntityType::Circle(good)).unwrap();
         let mut corrupt = acadrust::entities::Circle::new();
         corrupt.center = acadrust::types::Vector3::new(1.0, 1.0, 0.0);
-        corrupt.radius = 0.0; // io::is_entity_corrupt rejects a zero-radius circle
+        // Past any drawing's extent: io::is_entity_corrupt treats it as parser
+        // desync. (A zero radius is legal geometry and survives the purge.)
+        corrupt.radius = 1.0e11;
         doc.add_entity(acadrust::EntityType::Circle(corrupt)).unwrap();
         let bytes = crate::io::save_to_bytes(&doc, "dxf", doc.version)
             .expect("save a document containing a corrupt entity");