basicwidget: point SetBold at Weight for reading

TextStyle.SetBold has no Bold getter, as bold is a point on the weight
axis rather than a boolean, and a getter would have to invent a
threshold for which weights count as bold. Note in its doc comment
that TextStyle.Weight reads the weight back, alongside the existing
pointer at UnsetWeight for unsetting it.

Updates #131

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: hajimehoshi

basicwidget/textstyle.go

@@ -118,7 +118,8 @@ func (s *TextStyle) Weight() (text.Weight, bool) {
 }
 
 // SetBold sets the font weight to the bold or the medium weight by setting
-// the wght variation axis. Use [TextStyle.UnsetWeight] to unset the weight.
+// the wght variation axis. Use [TextStyle.UnsetWeight] to unset the weight
+// and [TextStyle.Weight] to read it back.
 func (s *TextStyle) SetBold(bold bool) {
 	weight := text.WeightMedium
 	if bold {