fix: DXF attribute rotation converted once; plugin layers refuse unknown linetypes

- The acadrust DXF reader has converted ATTRIB/ATTDEF rotation to radians
  since f226110 (in every codec rev OCS pinned from 2026-08-08 on), while
  fix_dxf_dimension_rotations still applied to_radians a second time on
  load. Only Shape needs the conversion here. Surfaced by #1391's hunk.
- Shape-file text styles are found by their is_shape_file flag instead of
  an empty name (also from #1391).
- #1393's add_layer/modify_layer wrote an unresolved linetype name into
  the LAYER record; a name the drawing (plus the standard set) cannot
  supply is now refused, through one resolve_linetype helper.

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

src/io/mod.rs

@@ -2368,21 +2368,13 @@ fn fix_viewport_status_flags(doc: &mut CadDocument) {
     }
 }
 
-/// The acadrust DXF reader stores several rotation fields directly from DXF
-/// group code 50 in degrees, while DWG and our own creation code store radians.
-/// Apply to_radians() on load so tessellation can call cos/sin uniformly.
+/// The acadrust DXF reader still stores Shape rotation directly from group
+/// code 50 in degrees, while DWG and our own creation code store radians.
+/// Dimension angles and ATTRIB/ATTDEF rotation are converted inside the
+/// reader, so arms for them here would convert twice.
 fn fix_dxf_dimension_rotations(doc: &mut CadDocument) {
     for entity in doc.entities_mut() {
         match entity {
-            // Dimension angles (rotation / text / oblique) are converted
-            // degrees->radians inside the acadrust DXF reader now, so a
-            // dimension arm here would double-convert.
-            EntityType::AttributeDefinition(a) => {
-                a.rotation = a.rotation.to_radians();
-            }
-            EntityType::AttributeEntity(a) => {
-                a.rotation = a.rotation.to_radians();
-            }
             EntityType::Shape(s) => {
                 s.rotation = s.rotation.to_radians();
             }