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/scene/mod.rs

@@ -40,6 +40,7 @@ mod modify;
 pub mod named_parameters;
 mod named_parameters_persist;
 mod mspace;
+pub mod viewport_ref;
 mod page_setup;
 mod paper;
 mod preview;