feat(snap): snap paper space through layout viewports

Paper-space commands could not pick model geometry displayed by a layout
viewport: `hit_test_wires` deliberately excludes viewport content in PSPACE,
and that has to stay true for selection. This adds the capability as a separate
query rather than by changing those semantics.

`OpenCADStudio::paper_viewport_snap` walks the current layout's displayed
content viewports top-most first, skips those whose display is off or whose
clip boundary excludes the cursor, and for each one runs the EXISTING snap
engine against that viewport's model wires using the viewport's own camera and
screen rectangle (`viewport_edit_frame_for`). The accepted model point is
mapped back onto the sheet with `ViewportFrame::model_to_paper`, so everything
downstream keeps working in paper coordinates — LINE still draws at the
projected paper point. Grid snap and the paper-space rubber-band origin are
suppressed for that pass; results outside the clip are rejected so no snap
lands on a clip cut. Locked viewports snap like unlocked ones.

No projected geometry is rebuilt per pointer event: the wire set is the
renderer's resident per-viewport Arc (`model_wires_for_viewport_arc`) and the
broad phase reuses the shared interaction index over it.

The result is merged with the ordinary paper-sheet snap by `snap::merge_snap`,
which applies the engine's own priority-then-distance rule, in both the
cursor-move and the click paths.

`SnapResult` gains Copy fields `viewport: Option<Handle>` and
`source: Option<DimensionAssociationSource>`; the engine fills `source` from
the owning wire wherever it knows it. `AcceptedSnap::from_snap` converts a
result plus frame into the shared PR2/PR3 contract type.

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

src/app/update/mod.rs

@@ -95,6 +95,7 @@ mod file;
 mod style;
 mod util;
 mod viewport;
+mod viewport_snap;
 
 impl OpenCADStudio {
     pub(in crate::app) fn reset_modal_geometry(&mut self) {