fix(web): gate unix-only signal + test helpers off the windows build

Windows clippy under -D warnings flagged two items as dead code: the
SIGNAL_RECEIVED atomic (read only by the cfg(unix) signal handler and
its watcher) and with_test_binary (called only from the cfg(unix) exit
tests, whose stand-ins are /bin/sh scripts). Both now carry the same
gate as their sole users, so they stay live-code on every platform.
The whole native test suite (9796 tests) is green on windows this
round; this clears the last clippy warning.

crates/op-host-services/src/chat_subprocess.rs

@@ -270,7 +270,10 @@ impl SubprocessProvider {
 
     /// Point a known-CLI provider at a stand-in binary so the exit /
     /// stderr / stdout handling can be exercised without the real CLI.
-    #[cfg(test)]
+    /// Its only callers are the unix-gated exit tests (the stand-ins are
+    /// `/bin/sh` scripts), so it carries the same gate to stay live-code
+    /// on Windows.
+    #[cfg(all(test, unix))]
     pub(crate) fn with_test_binary(mut self, binary: impl Into<String>) -> Self {
         self.binary = binary.into();
         self

crates/op-host-services/src/web_canvas_server/online_run_loop.rs

@@ -324,7 +324,9 @@ pub(super) fn check_persistence_configured(
 /// Raised by the signal handler. A `static` because that is all a handler may
 /// safely touch: it runs on an arbitrary thread with almost nothing allowed,
 /// so it sets one atomic and returns, and the watcher thread below does the
-/// rest.
+/// rest. Unix-only — the handler and its watcher are the only readers, and
+/// Windows keeps the supervisor-driven shutdown it has always had.
+#[cfg(unix)]
 static SIGNAL_RECEIVED: AtomicBool = AtomicBool::new(false);
 
 /// Route SIGTERM and SIGINT into the daemon's existing shutdown path.