Escape DGN text in MTEXT and read 3D text by its byte count

Backslashes and braces in a DGN text stay characters in the MTEXT it
becomes instead of turning into formatting codes.

A 3D V8 text is cut at its stored byte count before it is decoded, so a
multi-byte character no longer shifts where the text ends.

Doc comments that had slipped onto the wrong functions are back on
their own, comments describe the behaviour instead of pointing at
another program, and a few lines added in the previous commits are laid
out the way the rest of the code is.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/app/commands/dgn_import.rs

@@ -458,7 +458,8 @@ impl OpenCADStudio {
                 .file_stem()
                 .map(|s| s.to_string_lossy().into_owned())
                 .unwrap_or_default();
-            Naming::Prefix(dgn_import::symbol_name(&format!("{stem} - {}$0$", request.model)))
+            let prefix = format!("{stem} - {}$0$", request.model);
+            Naming::Prefix(dgn_import::symbol_name(&prefix))
         } else {
             Naming::Keep
         };