Feat: DIST / ID / AREA inquiry commands

Add three classic CAD measurement commands:
- DIST (DI): two-point pick → distance, angle, delta XYZ
- ID: one-point pick → coordinates
- AREA: multi-point polygon pick (Enter to close) → area + perimeter

New CmdResult::Measurement(String) variant ends active command and
prints result to command line without modifying the document.

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

src/command/mod.rs

@@ -93,6 +93,8 @@ pub enum CmdResult {
     PasteClipboard { base_pt: Vec3 },
     /// Zoom the model-space camera to fit the given corner points; end command.
     ZoomToWindow { p1: Vec3, p2: Vec3 },
+    /// Print a measurement result to the command line and end the command.
+    Measurement(String),
 }
 
 // ── Trait ─────────────────────────────────────────────────────────────────