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.

flake.nix

@@ -98,6 +98,8 @@
         checks.tests = craneLib.cargoTest (common // wasiCcEnv // {
           inherit cargoArtifacts;
           cargoExtraArgs = "--locked ${testPackages}";
+          # Stress tests are ignored in a normal `cargo test`. The check runs them.
+          cargoTestExtraArgs = "-- --include-ignored";
           # Nothing builds on the test run's target folder: don't store it.
           doInstallCargoArtifacts = false;
           # The SDK tests run sample plugins with the wasmtime CLI.