fix(snap): OSNAP Extension tracks only acquired endpoints

Extension snap extended every segment's line in the whole drawing, so in
a larger project an extension foot was live almost everywhere and the
cursor snapped to it constantly.

Extension now behaves like a temporary tracking point: an endpoint must
first be acquired by dwelling on it, and only then does that segment's
line extend. Acquisition — and the tracking-point marker — is active
whenever OTRACK is on OR the Extension snap is on, so Extension works
independently of OTRACK's on/off state (and is not tied to POLAR). Both
the per-segment extension and the extended-intersection candidates are
gated on an acquired endpoint, and the whole scan is skipped when nothing
is acquired (the common case), keeping large drawings responsive.

Closes #262

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.otrack_enabled {
+                if self.snapper.tracking_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)