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/host.rs

@@ -58,6 +58,7 @@ impl Host {
     /// Start a world with an explicit compile cache folder.
     pub async fn start_with_cache(path: &Path, cache: &Path) -> anyhow::Result<Self> {
         let path = path.to_path_buf();
+        let cargo_cache = cache.to_path_buf();
         let cache = cache.to_path_buf();
         let (world, runtime) = tokio::task::spawn_blocking(move || -> anyhow::Result<_> {
             Ok((World::open(&path)?, Runtime::new(&cache)?))
@@ -67,8 +68,10 @@ impl Host {
             journal::Journal::open(world.state_dir("core").join("journal.sqlite")).await?;
         let host = Self::with_parts(world, runtime, journal);
         let cargo_world = host.0.world.clone();
-        let cargo =
-            tokio::task::spawn_blocking(move || crate::cargo::Cargo::new(cargo_world)).await??;
+        let cargo = tokio::task::spawn_blocking(move || {
+            crate::cargo::Cargo::new(cargo_world, &cargo_cache)
+        })
+        .await??;
         cargo.register(&host);
         host.boot().await?;
         Ok(host)