runtime: name scheduler threads smarm-sched-{slot}

Extra scheduler threads (slots 1..N-1) are now spawned via thread::Builder
with the name smarm-sched-{slot}, so they are identifiable in
/proc/<pid>/task/*/comm, stack dumps and debuggers. Thread 0 keeps its
caller-given name (an embedder names the thread that calls run — smarm_beam
names it smarm-runtime). A refused spawn still panics, matching the previous
thread::spawn semantics.

Motivation: the §17 scheduler-width knob in smarm_beam asserts the *live*
width by counting these named threads, and a 20-scheduler soak needs the
threads tellable apart in wedge captures.

src/runtime.rs

@@ -967,16 +967,27 @@ impl Runtime {
         self.inner.root_swept.store(false, Ordering::Relaxed);
         self.inner.set_root(initial_handle.pid());
 
-        // Launch N-1 extra scheduler threads. The calling thread is thread 0.
+        // Launch N-1 extra scheduler threads, named `smarm-sched-{slot}` so
+        // they are identifiable in `/proc/<pid>/task/*/comm`, stack dumps and
+        // debuggers. The calling thread is thread 0 and keeps its caller-given
+        // name (an embedder typically names it when spawning `run`).
         let mut os_threads = Vec::new();
         for slot in 1..self.thread_count {
             let inner = self.inner.clone();
-            let t = thread::spawn(move || {
+            let t = thread::Builder::new()
+                .name(format!("smarm-sched-{slot}"))
+                .spawn(move || {
                     RUNTIME.with(|r| *r.borrow_mut() = Some(inner.clone()));
                     SCHED_SLOT.with(|s| s.set(slot));
                     schedule_loop(&inner, slot);
                     RUNTIME.with(|r| *r.borrow_mut() = None);
                 });
+            // `thread::spawn` (the previous form) also panics when the OS
+            // refuses a thread, so this keeps the failure semantics.
+            let t = match t {
+                Ok(t) => t,
+                Err(e) => panic!("failed to spawn smarm scheduler thread: {e}"),
+            };
             os_threads.push(t);
         }