feat(bench): ka/close A/B matrix — plain_serve baseline + box orchestrator

The jar's ka-vs-close throughput inversion was observed on the box but its
run configuration died with the job workspaces, so it can't be re-derived —
this commit makes the comparison a committed, controlled experiment instead
of a lost one-off.

- examples/plain_serve: the load_profile request path with zero causal
  machinery (same route/handler/Config), serving until killed. The plain
  half of the A/B; throughput measured externally by wrk.
- scripts/ka-close-matrix.sh: {ka,close} x {plain,causal} on fresh ports,
  byte-identical wrk args per mode pair except the Connection: close
  header, disjoint-core pinning for server vs wrk, per-cell curl
  verification of the negotiated connection behavior, ss/env capture, and
  a parsed summary with a close/ka ratio verdict. The causal cells double
  as the forgiveness-fix box validation (v13 pending item, pull-forward
  agreed): key on forgiven staying ~ injected x parked-actors in
  close-mode churn, not process-lifetime phantoms.

examples/plain_serve.rs

@@ -0,0 +1,35 @@
+//! Plain benchmark server: the `load_profile` request path with zero causal
+//! machinery — the baseline half of the ka/close A/B matrix (the
+//! throughput-inversion chase).
+//!
+//! Identical route, handler, and `Config` construction to `load_profile`;
+//! differs only in having no `smarm-causal` feature, no sweep, and no
+//! shutdown path — it serves until killed. Throughput is measured
+//! externally (wrk).
+//!
+//! Env:
+//!   URUS_PORT   listen port (default 8080; binds 0.0.0.0)
+//!
+//!   cargo run --release --example plain_serve
+
+use std::net::SocketAddr;
+
+use urus::{serve_with, Config, Conn, Next, Pipeline, Router};
+
+/// Mirrors `load_profile`'s handler byte for byte: param parse + JSON render.
+fn json_id(conn: Conn, _next: Next) -> Conn {
+    let id: u64 = conn.params.get("id").and_then(|s| s.parse().ok()).unwrap_or(0);
+    conn.put_status(200)
+        .put_header("content-type", "application/json")
+        .put_body(format!("{{\"id\":{id}}}"))
+}
+
+fn main() {
+    let port: u16 = std::env::var("URUS_PORT")
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(8080);
+    let addr: SocketAddr = format!("0.0.0.0:{port}").parse().expect("addr");
+    let pipe = Pipeline::new().plug(Router::new().get("/json/:id", json_id));
+    serve_with(Config::new(addr), pipe).expect("serve");
+}