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/notes/src/main.rs

@@ -0,0 +1,37 @@
+//! Keeps a list of notes in /data/notes.json and publishes them as
+//! /public/notes.txt.
+use serde_json::{Value, json};
+use std::io::{BufRead, Write};
+
+const DATA: &str = "/data/notes.json";
+
+fn save(notes: &[String]) {
+    std::fs::write(DATA, serde_json::to_string(notes).unwrap()).unwrap();
+    std::fs::write("/public/notes.txt", notes.join("\n") + "\n").unwrap();
+}
+
+fn main() {
+    let mut notes: Vec<String> = std::fs::read_to_string(DATA)
+        .ok()
+        .and_then(|s| serde_json::from_str(&s).ok())
+        .unwrap_or_default();
+    let mut out = std::io::stdout().lock();
+    for line in std::io::stdin().lock().lines() {
+        let Ok(line) = line else { break };
+        let Ok(msg) = serde_json::from_str::<Value>(&line) else { continue };
+        let body = &msg["body"];
+        let reply = match body["type"].as_str() {
+            Some("shutdown") => break,
+            Some("add") => {
+                notes.push(body["text"].as_str().unwrap_or_default().to_string());
+                save(&notes);
+                json!({"type": "added", "count": notes.len()})
+            }
+            Some("list") => json!({"type": "notes", "items": notes}),
+            Some("crash") => panic!("asked to crash"),
+            _ => json!({"type": "error", "reason": "unknown message"}),
+        };
+        writeln!(out, "{}", json!({"to": msg["from"], "reply_to": msg["id"], "body": reply})).unwrap();
+        out.flush().unwrap();
+    }
+}