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/fstest/Cargo.toml

@@ -0,0 +1,7 @@
+[package]
+name = "fstest"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+serde_json = "1"

examples/fstest/src/main.rs

@@ -0,0 +1,47 @@
+//! Filesystem probe for the spike. Keeps files open across messages.
+use serde_json::{Value, json};
+use std::collections::HashMap;
+use std::io::{BufRead, Write};
+
+fn main() {
+    let mut open: HashMap<String, std::fs::File> = HashMap::new();
+    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 b = &msg["body"];
+        let path = b["path"].as_str().unwrap_or_default().to_string();
+        let r = |res: std::io::Result<Value>| match res {
+            Ok(v) => v,
+            Err(e) => json!({"type": "error", "error": e.to_string()}),
+        };
+        let reply = match b["type"].as_str() {
+            Some("shutdown") => break,
+            Some("write") => r(std::fs::write(&path, b["text"].as_str().unwrap_or_default()).map(|_| json!({"type": "ok"}))),
+            Some("open") => r((|| {
+                let mut f = std::fs::File::create(&path)?;
+                f.write_all(b["text"].as_str().unwrap_or_default().as_bytes())?;
+                open.insert(path.clone(), f);
+                Ok(json!({"type": "ok", "open": true}))
+            })()),
+            Some("append") => r((|| {
+                let mut f = std::fs::OpenOptions::new().append(true).open(&path)?;
+                f.write_all(b["text"].as_str().unwrap_or_default().as_bytes())?;
+                open.insert(path.clone(), f);
+                Ok(json!({"type": "ok", "open": true}))
+            })()),
+            Some("close") => json!({"type": "ok", "closed": open.remove(&path).is_some()}),
+            Some("read") => r(std::fs::read_to_string(&path).map(|t| json!({"type": "content", "text": t}))),
+            Some("list") => r(std::fs::read_dir(&path).map(|rd| {
+                let mut names: Vec<String> = rd.filter_map(|e| e.ok()).map(|e| e.file_name().to_string_lossy().to_string()).collect();
+                names.sort();
+                json!({"type": "entries", "names": names})
+            })),
+            Some("remove") => r(std::fs::remove_file(&path).map(|_| json!({"type": "ok"}))),
+            Some("crash") => panic!("asked to crash"),
+            _ => json!({"type": "error", "error": "unknown message"}),
+        };
+        writeln!(out, "{}", json!({"to": msg["from"], "reply_to": msg["id"], "body": reply})).unwrap();
+        out.flush().unwrap();
+    }
+}