Add shared viewport frame, accepted snap, and measurement scale interfaces

Contract module for the viewport snapping, viewport dimension measurement,
and dimension association work that builds on it.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

src/command.rs

@@ -179,7 +179,7 @@ pub struct SelectionEntity {
 }
 
 /// Association source with an optional sub-entity marker.
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy, Debug, PartialEq)]
 pub struct DimensionAssociationSource {
     pub handle: Handle,
     pub marker: Option<i32>,