feat: ATTEDIT — edit block attribute values interactively

- attedit.rs: AtteditCommand picks an INSERT entity then iterates through
  each attribute tag prompting for a new value (Enter keeps existing)
- attedit_pending_handle() / attedit_set_attrs() trait methods inject
  attribute data into the command after entity-pick (avoids document access
  in the command itself)
- cmd_result.rs: NeedPoint handler injects attr data; ReplaceEntity detects
  the __ATTEDIT__ sentinel and calls apply_attedit() on the document
- apply_attedit() encodes tag/value pairs as __ATTEDIT__tag\x01val\x02...
- ROADMAP.md: ATTEDIT marked ✅

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

src/command/mod.rs

@@ -233,6 +233,16 @@ pub trait CadCommand: Send {
         None
     }
 
+    /// If this command needs attribute data injected (ATTEDIT), returns the
+    /// INSERT handle awaiting attr initialization; else None.
+    fn attedit_pending_handle(&self) -> Option<acadrust::Handle> {
+        None
+    }
+
+    /// Inject attribute (tag, value) pairs into the command after entity pick.
+    fn attedit_set_attrs(&mut self, _attrs: Vec<(String, String)>) {}
+
+
     /// Called instead of `on_point` when the command needs a tangent pick
     /// and the snap system found a tangent object.
     fn on_tangent_point(&mut self, obj: TangentObject, hit: Vec3) -> CmdResult {