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/runtime.rs

@@ -222,7 +222,8 @@ impl Runtime {
         let mut config = Config::new();
         config.epoch_interruption(true);
         let disk_cache = (|| -> Result<_> {
-            std::fs::create_dir_all(cache)?;
+            let cache = cache.join("wasmtime");
+            std::fs::create_dir_all(&cache)?;
             let mut cache_config = wasmtime::CacheConfig::new();
             cache_config.with_directory(cache.canonicalize()?);
             Ok(wasmtime::Cache::new(cache_config)?)