feat(bench): URUS_SCHED_THREADS knob in plain_serve

E1 (herd-vs-stranding discrimination) sweeps smarm scheduler count at
fixed low concurrency. Strict parse — a malformed value panics instead
of silently reverting to default, and the effective value is logged to
stderr so every bench cell's server.log records it (same per-cell
verification discipline as mode-verify).

examples/plain_serve.rs

@@ -8,7 +8,11 @@
 //! externally (wrk).
 //!
 //! Env:
-//!   URUS_PORT   listen port (default 8080; binds 0.0.0.0)
+//!   URUS_PORT           listen port (default 8080; binds 0.0.0.0)
+//!   URUS_SCHED_THREADS  smarm scheduler OS threads (default: smarm's own
+//!                       default). Malformed values panic rather than
+//!                       silently falling back — a benchmark knob that
+//!                       quietly reverts to default poisons the cell.
 //!
 //!   cargo run --release --example plain_serve
 
@@ -30,6 +34,15 @@ fn main() {
         .and_then(|v| v.parse().ok())
         .unwrap_or(8080);
     let addr: SocketAddr = format!("0.0.0.0:{port}").parse().expect("addr");
+    let sched_threads: Option<usize> = std::env::var("URUS_SCHED_THREADS").ok().map(|v| {
+        v.parse()
+            .unwrap_or_else(|_| panic!("URUS_SCHED_THREADS not a usize: {v:?}"))
+    });
+    // Audit line: lands in each bench cell's server.log so the effective
+    // scheduler count is recorded per cell, same discipline as mode-verify.
+    eprintln!("plain_serve: scheduler_threads={sched_threads:?}");
+    let mut cfg = Config::new(addr);
+    cfg.scheduler_threads = sched_threads;
     let pipe = Pipeline::new().plug(Router::new().get("/json/:id", json_id));
-    serve_with(Config::new(addr), pipe).expect("serve");
+    serve_with(cfg, pipe).expect("serve");
 }