review(#1660, #1663): associative sweep scan, arc angles, viewport off bit

- refresh_associative_sweeps runs on every edit and scanned the xdata of
  every entity; it now returns at once when no associative swept surface
  exists (its application is not registered) and looks at surfaces only.
- An arc's end angle was raised by TAU in a loop, which never ends on a
  huge angle from a damaged file; the sweep is taken with rem_euclid.
- The sweep DELOBJ test expected surface sweeps to delete their sources,
  which #1663 changed on purpose: an associative surface keeps them.
- codec c9ffcac (opencadcodec #103) adds an explicit viewport-off bit: the
  viewport on state is read with is_on() and set with turn_on()/turn_off()
  everywhere, so a viewport the file marks off shows off and can be turned
  back on from Properties.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/viewport_dimension_tests.rs

@@ -27,7 +27,7 @@ fn fixture() -> (OpenCADStudio, Handle, ViewportFrame) {
     viewport.view_height = 1000.0;
     viewport.custom_scale = 0.1;
     viewport.view_direction = Vector3::UNIT_Z;
-    viewport.status.is_on = true;
+    viewport.turn_on();
     let viewport = scene.add_entity(EntityType::Viewport(viewport));
     let frame = scene.viewport_frame(viewport).unwrap();
     scene.document.header.dimension_associativity = 2;
@@ -484,7 +484,7 @@ fn viewport_dimension_eligibility_is_shared_and_uses_clipping() {
         first.viewport
     );
     if let Some(EntityType::Viewport(vp)) = scene.document.get_entity_mut(first.viewport) {
-        vp.status.is_on = false;
+        vp.turn_off();
     }
     assert!(scene.viewport_frames_at_paper_point(paper).is_empty());
     assert!(scene.dimension_pick_through_viewport(paper, 0.2).is_none());