feat: DDEDIT command and double-click text editing

Add DDEDIT (ED alias) command that edits Text/MText/Attribute entities
in-place via the command line. Double-clicking a text entity in model space
now auto-launches DDEDIT for that entity. Supports Text, MText,
AttributeDefinition, and AttributeEntity types.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/command/mod.rs

@@ -124,6 +124,8 @@ pub enum CmdResult {
     },
     /// Set the plot window on the active layout's PlotSettings.
     SetPlotWindow { p1: Vec3, p2: Vec3 },
+    /// Replace the text content of a Text/MText entity in-place.
+    DdeditEntity { handle: Handle, new_text: String },
 }
 
 // ── Trait ─────────────────────────────────────────────────────────────────