Keep the persisted representation and the seam tests consistent

Colocated unit tests pin the persisted representation: a 1:10 viewport with
no user factor writes DIMLFAC -10, a 10-unit paper span then formats as 100,
and the same dimension read back in model space does not scale. A positive
user factor folds into the magnitude, and a drawing-wide negative DIMLFAC
that already carries the compensation writes no override at all. Another
test pins the viewport model->paper affine against ViewportFrame's own
mapping, so object-pick projection and snap projection cannot drift.

Also clears the recorded snaps when a multi-dimension command commits.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

src/app/command_driver.rs

@@ -2254,6 +2254,7 @@ impl OpenCADStudio {
                 }
             }
             CmdResult::CommitDimensionsAndExit(dimensions) => {
+                self.clear_dimension_accepted_snaps();
                 let association_mode = self.tabs[i]
                     .scene
                     .document