Sausage: host, broker, commit-on-close filesystem, cargo actor, console, agent

Imported from the spike branch, with:
- a library crate (Host API) for the CLI and the tests
- JSON test scenarios for candidates
- commit on close only for new or truncated files
- a Nix flake: nix run -- init, nix run -- run

examples/bench/src/main.rs

@@ -0,0 +1,45 @@
+//! {"type":"bench","target":"echo","n":1000}: ping-pong with the target, one
+//! message at a time, and report the round-trip times.
+use serde_json::{Value, json};
+use std::io::{BufRead, Write};
+use std::time::Instant;
+
+fn main() {
+    let mut out = std::io::stdout().lock();
+    let mut lines = std::io::stdin().lock().lines();
+    while let Some(Ok(line)) = lines.next() {
+        let Ok(msg) = serde_json::from_str::<Value>(&line) else { continue };
+        match msg["body"]["type"].as_str() {
+            Some("shutdown") => break,
+            Some("bench") => {
+                let target = msg["body"]["target"].as_str().unwrap_or("echo").to_string();
+                let n = msg["body"]["n"].as_u64().unwrap_or(1000);
+                let mut times = Vec::new();
+                let started = Instant::now();
+                for i in 0..n {
+                    let t = Instant::now();
+                    writeln!(out, "{}", json!({"to": target, "id": i, "body": {"type": "ping", "i": i}})).unwrap();
+                    out.flush().unwrap();
+                    // Wait for the reply.
+                    loop {
+                        let Some(Ok(l)) = lines.next() else { return };
+                        let Ok(r) = serde_json::from_str::<Value>(&l) else { continue };
+                        if r["reply_to"].as_str().is_some_and(|s| s.ends_with(&format!("/{i}"))) {
+                            break;
+                        }
+                    }
+                    times.push(t.elapsed().as_micros() as u64);
+                }
+                let total = started.elapsed().as_secs_f64();
+                times.sort();
+                let p = |q: f64| times[((times.len() as f64 - 1.0) * q) as usize];
+                let reply = json!({"type": "bench_result", "n": n, "total_s": total,
+                    "msgs_per_s": (n as f64 / total).round(),
+                    "p50_us": p(0.5), "p90_us": p(0.9), "p99_us": p(0.99), "max_us": p(1.0)});
+                writeln!(out, "{}", json!({"to": msg["from"], "reply_to": msg["id"], "body": reply})).unwrap();
+                out.flush().unwrap();
+            }
+            _ => {}
+        }
+    }
+}