feat: DATAEXTRACTION — export entity data to CSV

- DATAEXTRACTION / EATTEXT / ATTEXT commands trigger an async save dialog
- Extracts all model-space entities with: Type, Handle, Layer, Color,
  Linetype, and a geometry summary per entity type (coords, radius, etc.)
- DataExtractionSave / DataExtractionSaveResult message pair drives
  the rfd save-file dialog asynchronously
- CSV fields are properly escaped (commas, quotes, newlines)
- ROADMAP.md: 12 DATAEXTRACTION marked ✅

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/mod.rs

@@ -422,6 +422,11 @@ pub enum Message {
     WblockSave(String),
     /// Result of the WBLOCK save path dialog.
     WblockSaveResult(String, Option<std::path::PathBuf>),
+    // ── DATAEXTRACTION ────────────────────────────────────────────────────
+    /// Save the pre-built CSV string to a file chosen by the user.
+    DataExtractionSave(String),
+    /// Path chosen (or None = cancelled).
+    DataExtractionSaveResult(String, Option<std::path::PathBuf>),
 }
 
 impl H7CAD {