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.

DECISIONS.md

@@ -69,6 +69,12 @@ a plugin, or a change that the agent makes to itself.
   core.
 - **The SDK is not published.** `cargo` points `sausage-plugin = "0.1"` at
   its own copy, which it also shows in `/plugins/cargo/sdk/`.
+- **Compiled code is a cache, not state.** It lives outside the world, in
+  one cache folder that all worlds share: `wasmtime/` (owned by Wasmtime,
+  whose cleanup deletes unknown entries), `cargo-target/`, and
+  `sdk/<hash>/`. A path that depends only on the content of the SDK lets
+  cargo reuse the compiled SDK in every world. Tests run in the release
+  profile, so that they share compiled crates with the release build.
 - **Discovery has a deadline of 10 seconds.** It is an internal call of the
   core, not a limit on plugins. Without it, one plugin that does not answer
   would block every activation.