Draw only the first character of a multi-character bullet

PowerPoint draws buChar char="••" (seen in a LibreOffice test deck) as a
single dot; DeckCraft drew both, pushing the text right.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/text/src/lib.rs

@@ -679,7 +679,8 @@ fn map_symbol_bullet(c: &str, font: Option<&str>) -> String {
         };
         return m.to_string();
     }
-    c.to_string()
+    // PowerPoint draws only the first character of a longer `buChar` ("••" draws one dot).
+    c.graphemes(true).next().unwrap_or("•").to_string()
 }
 
 fn measure_str(face: &FontFace, size: f64, s: &str) -> (f64, Vec<(u32, f64)>) {