feat(snap): parallel object snap (#277)

Parallel snap did nothing — the type was in the osnap set, priority and
marker, but there was no logic behind it. Implement it: hovering a line
(or polyline segment) for a short dwell acquires its direction as a
reference; then while drawing from the last point, when the cursor runs
parallel to that reference the point locks onto the line through the last
point parallel to it, showing the ∥ marker and the alignment guide.
Curves (circle/arc/ellipse) are ignored — parallel to a curve is
undefined. Works with only the Parallel object snap on, independent of
OTRACK; the reference clears with the command's tracking state.

Closes #277

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/view/mod.rs

@@ -340,7 +340,7 @@ impl OpenCADStudio {
             // Shared OTRACK projection basis: the active pane's camera + rect
             // (canvas offset included), matching the grips / UCS icon above.
             let otrack_proj: Option<(glam::Mat4, glam::DVec3, iced::Rectangle)> =
-                if self.snapper.tracking_active() {
+                if self.snapper.alignment_active() {
                     Some(
                         if let Some((vp_cam, full)) = tab.scene.viewport_edit_frame((vw, vh)) {
                             (vp_cam.view_proj_rte(full), vp_cam.eye(), full)