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 <noreply@anthropic.com>

src/entities/shape.rs

@@ -119,7 +119,7 @@ fn shx_polylines(
     //    glyph named like this shape.
     if !name.is_empty() {
         for style in document.text_styles.iter() {
-            if style.name.is_empty() && !style.font_file.trim().is_empty() {
+            if style.is_shape_file && !style.font_file.trim().is_empty() {
                 if let Some(p) = try_style(style) {
                     return Some(p);
                 }