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/crasher/Cargo.toml

@@ -0,0 +1,6 @@
+[package]
+name = "crasher"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]

examples/crasher/src/main.rs

@@ -0,0 +1,6 @@
+//! A bad version: it starts, waits 3 seconds (long enough for the smoke
+//! test), then panics. Used to test the automatic rollback.
+fn main() {
+    std::thread::sleep(std::time::Duration::from_secs(3));
+    panic!("this version is broken");
+}