feat(mtext): structured editor with selection, copy, and visible bold

Rewrite the in-app MTEXT editor to run on acadrust's structured
MTextDocument model end-to-end: load via parse_mtext, edit a flat Cell
model, serialize via to_mtext_string. Retires the hand-rolled format-code
string handling, the parallel visible_spans parser, the composed_value
prefix, and the Edit-raw-code tab.

Selection and copy:
- double-click selects the word, triple-click selects all (click-counting)
- Ctrl+A selects all text
- Ctrl+C copies the selected text to the clipboard

Formatting:
- per-selection Bold/Italic/Underline/Overline/Strike/Case/Alignment
- Bold renders as a visibly thicker SDF pen, keyed (family, char, bold)
  through the whole text pipeline; the resolved style font name is stamped
  so the \f...|b1; code survives serialization
- per-selection font and color changes
- editor color dropdown reuses the Properties panel color selector

Global toolbar defaults (font/color/oblique/width/tracking) fold onto the
document as real span properties instead of a leading code prefix; a
trailing empty paragraph is preserved so Enter at the end keeps its line.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/entities/attribute.rs

@@ -306,6 +306,7 @@ fn build_attr_truck(input: AttrTextInputs<'_>, document: &acadrust::CadDocument)
                 width_factor,
                 oblique: oblique_angle,
                 tracking: 1.0,
+                bold: false,
             }),
         });
     }

src/entities/multileader.rs

@@ -1529,6 +1529,7 @@ impl MultiLeaderTess for MultiLeader {
                             run.oblique,
                             run.tracking,
                             &run.font,
+                            run.bold,
                             &run.text,
                         );
                         crate::scene::pipeline::text_gpu::push_glyph_vertices(

src/entities/table.rs

@@ -724,6 +724,7 @@ pub fn tessellate_table(
                         run.oblique,
                         run.tracking,
                         &run.font,
+                        run.bold,
                         &run.text,
                     );
                     crate::scene::pipeline::text_gpu::push_glyph_vertices(

src/entities/text.rs

@@ -123,6 +123,7 @@ fn to_truck(t: &Text, document: &acadrust::CadDocument) -> TruckEntity {
                 width_factor: p.width_factor,
                 oblique: p.oblique_angle,
                 tracking: 1.0,
+                bold: false,
             }),
         }]),
         snap_pts: vec![(snap_pt, SnapHint::Insertion)],

src/entities/text_support.rs

@@ -275,6 +275,8 @@ pub struct RunState {
     pub underline: bool,
     pub overline: bool,
     pub strike: bool,
+    /// Bold run — rendered with a wider SDF pen (thicker strokes).
+    pub bold: bool,
 }
 
 impl Default for RunState {
@@ -290,6 +292,7 @@ impl Default for RunState {
             underline: false,
             overline: false,
             strike: false,
+            bold: false,
         }
     }
 }
@@ -435,6 +438,7 @@ pub fn adapt_mtext_paragraphs(
                     _ => 0,
                 },
                 font: p.font.as_ref().map(|f| font_stem(&f.name)),
+                bold: p.font.as_ref().map(|f| f.bold).unwrap_or(false),
                 color: color_of(p),
                 underline: p.stroke.underline(),
                 overline: p.stroke.overline(),
@@ -1108,6 +1112,7 @@ pub fn layout_mtext(opts: &MTextRenderOpts) -> MTextLayout {
                             width_factor: signed_wf,
                             oblique,
                             tracking,
+                            bold: atom.state.bold,
                         }),
                     });
                     if opts.want_glyph_boxes {

src/entities/tolerance.rs

@@ -289,6 +289,7 @@ impl TruckConvertible for Tolerance {
                     width_factor: 1.0,
                     oblique: 0.0,
                     tracking: 0.0,
+                    bold: false,
                 }),
             });
         }