refactor(snap): measure typed distance along the guide that is drawn

Extension answered "which acquired ray is this snap on?" twice, from two
different tests: the overlay picked its guide by off-ray distance in
screen pixels, while typed-distance entry re-searched the tracking set in
world space. Two tests can disagree, and the ways they disagree are both
bad -- a guide on screen that typing ignores, or a distance running off
along a ray the user cannot see. The second search could also come up
empty on its own, since only four acquisitions are kept and the oldest is
dropped as new ones arrive.

Ask once. The search that draws the guide now also reports the endpoint it
drew from, the snap carries it, and typed distance reads the ray straight
off that -- the direction is just the way from the endpoint to the snap,
which sits beyond it by construction. The screen-space test stays the only
one, which is what the guide wanted anyway: it stays scale-independent
where a world test would not.

Also correct the tracking-point doc: it claims XZ with Y=0, but
`edge_dirs_at` builds every direction from X/Y with Z zeroed and the rays
follow it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/app/update/viewport.rs

@@ -1647,6 +1647,7 @@ impl OpenCADStudio {
                         tangent_obj: None,
                         extension_base: None,
                         extension_base2: None,
+                        extension_origin: None,
                     });
                     if let Some(cmd) = self.tabs[i].active_cmd.as_mut() {
                         cmd.set_acquisition_hint(Some(pick.label));