fix: mitigate tokio I/O driver starvation (tokio-rs/tokio#4730)

Tokio's multi-thread scheduler has an unfixed vulnerability where all
worker threads can end up parked on condvars with no worker driving the
I/O reactor.  Condvar-parked workers have no timeout and sleep
indefinitely, so once in this state the runtime never recovers.

This was observed on a box migrating from 0.3.5.1: after heavy task
churn (package reinstalls, container operations, logging) all 16 workers
ended up on futex_wait with no thread on epoll_wait.  The web server
listened on both HTTP and HTTPS but never replied.  The box was stuck
for 7+ hours with 0% CPU.

Two mitigations:

1. Watchdog OS thread (startd.rs): a plain std::thread that every 30s
   injects a no-op task via Handle::spawn.  This forces a condvar-parked
   worker to wake, cycle through park, and grab the driver TryLock —
   breaking the stall regardless of what triggered it.

2. block_in_place in the logger (logger.rs): the TeeWriter holds a
   std::sync::Mutex across blocking file + stderr writes on worker
   threads.  Wrapping in block_in_place tells tokio to hand off driver
   duties before the worker blocks, reducing the window for starvation.
   Guarded by runtime_flavor() to avoid panicking on current-thread
   runtimes used by the CLI.

core/src/bins/startd.rs

@@ -148,6 +148,15 @@ pub fn main(args: impl IntoIterator<Item = OsString>) {
             .build()
             .expect(&t!("bins.startd.failed-to-initialize-runtime"));
         let res = rt.block_on(async {
+            // Periodically wake a worker thread from a non-tokio OS thread to
+            // prevent tokio I/O driver starvation (all workers parked on
+            // condvar with no driver).  See tokio-rs/tokio#4730.
+            let rt_handle = tokio::runtime::Handle::current();
+            std::thread::spawn(move || loop {
+                std::thread::sleep(Duration::from_secs(30));
+                rt_handle.spawn(async {});
+            });
+
             let mut server = WebServer::new(Acceptor::new(WildcardListener::new(80)?), refresher());
             match inner_main(&mut server, &config).await {
                 Ok(a) => {