feat(app): record accepted snaps per point step and guard dimension inference

Point input now keeps an `AcceptedSnap` alongside each point a command
collects: the paper point, the model point it came from, the viewport and
frame it was seen through, and the geometry identity the snap engine
attributed. Interactive picks record in the click handler, typed / dynamic /
headless points record in `feed_command`, and the list is cleared whenever a
command starts or the active space changes. Commands that don't care keep
consuming the plain paper point unchanged; PR2 reads the model point and
frame, PR3 the source.

Dimension association inference is suppressed when the dimension's points came
through a viewport. Inference matches geometry against the dimension's own
definition points in whatever space it is handed, so paper definition points
projected from a viewport would associate with unrelated paper-sheet geometry
at those coordinates, or match model geometry using paper coordinates. An
empty source list makes `attach_dimension_association` a no-op. PR3 replaces
this with real viewport-aware associations.

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

src/app/commands/mod.rs

@@ -120,6 +120,9 @@ impl OpenCADStudio {
         // Reset the last committed point so the first click of the new command
         // is not constrained by ortho/polar relative to a previous command's endpoint.
         self.last_point = None;
+        // A new command collects its own points, so the previous command's
+        // accepted snaps must not leak into it (PR1).
+        self.clear_accepted_snaps();
         // Starting a command restarts the right-click cycle, so its first
         // right-click acts as Enter rather than opening the context menu.
         self.tabs[i]