fix(plugin): f64 pick coords + harden out-of-process lifecycle

Follow-ups after merging the out-of-process plugin isolation work:

- Convert the out-of-process interactive adapter's `on_point` /
  `on_entity_pick` to `glam::DVec3`, matching the `CadCommand` trait,
  which moved to f64 pick coordinates. The branch predated that change
  and no longer compiled against the trait.
- Contain plugin constructor panics in `export_plugin!`: the constructor
  runs across the C ABI boundary in `ocs_plugin_register`, where an
  unwinding panic is undefined behavior. Catch it and return null, which
  the loader already treats as a failed registration.
- Reap killed runner processes. The spawn-timeout and disconnect paths
  called `child.kill()` without `wait()`, leaving a zombie on Unix until
  the host exited. Route both — and `shutdown()` — through a shared
  `reap()` that kills and waits on a detached thread.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/ocs_plugin_api/src/host.rs

@@ -97,9 +97,18 @@ macro_rules! export_plugin {
         #[no_mangle]
         pub extern "C" fn ocs_plugin_register(
         ) -> *mut ::std::boxed::Box<dyn $crate::host::BuiltinPlugin> {
-            let plugin: ::std::boxed::Box<dyn $crate::host::BuiltinPlugin> =
-                ::std::boxed::Box::new($ctor);
-            ::std::boxed::Box::into_raw(::std::boxed::Box::new(plugin))
+            // The constructor runs across a C ABI boundary; a panic unwinding
+            // past it is undefined behavior. Contain it and report failure as a
+            // null pointer, which the host loader treats as "registration
+            // failed" rather than crashing the runner process.
+            match ::std::panic::catch_unwind(::std::panic::AssertUnwindSafe(|| {
+                let plugin: ::std::boxed::Box<dyn $crate::host::BuiltinPlugin> =
+                    ::std::boxed::Box::new($ctor);
+                ::std::boxed::Box::into_raw(::std::boxed::Box::new(plugin))
+            })) {
+                ::std::result::Result::Ok(ptr) => ptr,
+                ::std::result::Result::Err(_) => ::std::ptr::null_mut(),
+            }
         }
     };
 }