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 <[email protected]>

src/io/mod.rs

@@ -2539,7 +2539,7 @@ fn fix_viewport_status_flags(doc: &mut CadDocument) {
             // If bit 0 is not set but bit 15 is, this is an external-format viewport:
             // treat bit 15 as "on" and bit 14 as "locked".
             if (bits & 0x0001) == 0 && (bits & 0x8000) != 0 {
-                vp.status.is_on = true;
+                vp.turn_on();
                 vp.status.locked = (bits & 0x4000) != 0;
             }
         }