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/ticker/src/main.rs

@@ -0,0 +1,16 @@
+//! Sends {"type":"add","text":"tick N"} to `notes` every 50 ms, N = 1..=200.
+//! Does not read stdin, so the broker kills it at shutdown; that is fine.
+use std::io::Write;
+fn main() {
+    let mut out = std::io::stdout().lock();
+    for n in 1..=200 {
+        let msg = serde_json::json!({"to": "notes", "body": {"type": "add", "text": format!("tick {n}")}});
+        writeln!(out, "{msg}").unwrap();
+        out.flush().unwrap();
+        std::thread::sleep(std::time::Duration::from_millis(50));
+    }
+    // Stay alive without doing anything.
+    loop {
+        std::thread::sleep(std::time::Duration::from_secs(3600));
+    }
+}