fix(text): resolve %%c to U+2205, the diameter sign AutoCAD writes

Saved dimension pictures across 1311 diameter dimensions carry U+2205 (806) or a
literal %%c (168); U+2300 never appears. The bundled romans and simplex fonts
have U+2205 and lack Latin O-stroke, so one character now serves the %%c
expansion and, in the dimension formatter, the diameter prefix.

src/entities/text_support.rs

@@ -197,7 +197,7 @@ pub fn text_local_bounds(
 /// Expand DXF `%%x` special-character sequences that appear in both TEXT and MTEXT values:
 /// - `%%d` / `%%D` → `°`
 /// - `%%p` / `%%P` → `±`
-/// - `%%c` / `%%C` → `⌀`
+/// - `%%c` / `%%C` → `∅` (U+2205, the character AutoCAD writes for the diameter sign)
 /// - `%%u` / `%%U` → underline toggle (stripped — not renderable with stroke fonts)
 /// - `%%o` / `%%O` → overline toggle (stripped)
 /// - `%%%%` → `%`
@@ -224,7 +224,7 @@ pub fn resolve_dxf_special_chars(s: &str) -> String {
             }
             Some('c') => {
                 chars.next();
-                out.push('⌀');
+                out.push('∅');
             }
             Some('u') | Some('o') => {
                 chars.next();