cargo.build: a persistent build cache shared by all worlds

- The cargo target folder is <cache>/cargo-target, next to Wasmtime's
  cache, which moves to <cache>/wasmtime: Wasmtime's cleanup worker
  deletes unknown entries in its folder, also Cargo's artifacts.
- Builds use the SDK at <cache>/sdk/<content hash>/, so cargo reuses
  the compiled SDK in every world. /plugins/cargo/sdk/ stays for agents.
- Tests run in the release profile and share compiled crates with the
  release build.
- The cargo and cli tests use a persistent cache in CARGO_TARGET_TMPDIR:
  warm, the cargo suite takes 11 s (was 235 s) and the cli suite 6 s
  (was 149 s).
- nix flake check runs ignored stress tests. AGENTS.md: rules for fast
  tests.

cli/src/main.rs

@@ -79,7 +79,7 @@ fn setting(var: &str, question: &str, default: Option<&str>) -> Result<String> {
     }
 }
 /// Creates a configured world and builds its embedded agent through the standalone builder.
-async fn init(dir: &Path) -> Result<()> {
+async fn init(dir: &Path, cache: Option<&Path>) -> Result<()> {
     let api = setting("SAUSAGE_API", "API shape", Some("openai-responses"))?;
     if api != "openai-responses" && api != "anthropic-messages" {
         bail!("unknown API shape {api:?}");
@@ -105,7 +105,15 @@ async fn init(dir: &Path) -> Result<()> {
             &json!({"api":api,"base_url":base_url.trim_end_matches('/'),"model":model,"token":token}),
         )?,
     )?;
-    let builder = sausage::cargo::Cargo::new(world.clone())?;
+    let cache = cache.map(Path::to_path_buf).unwrap_or_else(|| {
+        let base = std::env::var_os("XDG_CACHE_HOME")
+            .map(PathBuf::from)
+            .unwrap_or_else(|| {
+                PathBuf::from(std::env::var_os("HOME").unwrap_or_default()).join(".cache")
+            });
+        base.join("sausage")
+    });
+    let builder = sausage::cargo::Cargo::new(world.clone(), &cache)?;
     let source = world.plugin_dir("agent").join("source");
     std::fs::create_dir_all(source.join("src"))?;
     AGENT.extract(source.join("src"))?;
@@ -133,7 +141,7 @@ async fn main() -> Result<std::process::ExitCode> {
         None => {
             let dir = PathBuf::from("world");
             if !dir.join("config/boot.json").is_file() {
-                init(&dir).await?;
+                init(&dir, cli.cache_dir.as_deref()).await?;
             }
             Cmd::Run { dir }
         }
@@ -144,7 +152,7 @@ async fn main() -> Result<std::process::ExitCode> {
                 return Ok(std::process::ExitCode::FAILURE);
             }
         }
-        Cmd::Init { dir } => init(&dir).await?,
+        Cmd::Init { dir } => init(&dir, cli.cache_dir.as_deref()).await?,
         Cmd::Run { dir } => {
             let mut term =
                 tokio::signal::unix::signal(tokio::signal::unix::SignalKind::terminate())?;