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 <noreply@anthropic.com>

src/app/mod.rs

@@ -538,6 +538,12 @@ pub(super) struct OpenCADStudio {
     last_vp_click_time: Option<Instant>,
     /// Screen position of the previous viewport left-click release.
     last_vp_click_pos: Option<Point>,
+    /// MText preview click tracking for double-click (select word) / triple-click
+    /// (select all): the previous click's time + visible offset, and the run
+    /// length of quick same-spot clicks (1..=3).
+    mtext_click_time: Option<Instant>,
+    mtext_click_off: usize,
+    mtext_click_count: u8,
     /// Plot scale for model-space window plots: "Fit" | "1:1" | … | "2:1".
     plot_scale: String,
     /// Pending model-space plot window (x0, y0, x1, y1) in world XY, or None.
@@ -789,6 +795,8 @@ pub enum ColorPickTarget {
     Ribbon,
     /// A layer's colour, by panel row index.
     Layer(usize),
+    /// The MText editor's selection (or global) colour.
+    MText,
 }
 
 /// Table records the clipboard entities depend on, snapshotted from the source
@@ -1758,8 +1766,11 @@ pub enum Message {
     MTextWidth(String),
     /// Toolbar character-spacing field changed.
     MTextCharSpace(String),
-    /// Toolbar colour dropdown — global text colour (ACI index, 256 = ByLayer).
-    MTextColor(u16),
+    /// Toolbar colour picker (same widget as Properties) — applies to the
+    /// selection, or the whole text when nothing is selected.
+    MTextColorChanged(AcadColor),
+    /// Open / close the MText colour picker popup.
+    MTextColorPickerToggle,
     /// Toolbar justification / attachment-point change.
     MTextJustify(acadrust::entities::mtext::AttachmentPoint),
     /// Toolbar paragraph-alignment change.
@@ -2194,6 +2205,9 @@ impl OpenCADStudio {
             layout_rename_state: None,
             last_vp_click_time: None,
             last_vp_click_pos: None,
+            mtext_click_time: None,
+            mtext_click_off: 0,
+            mtext_click_count: 0,
             plot_scale: "Fit".to_string(),
             plot_window: None,
             plot_format: crate::io::paper_sizes::PaperSize::A4,