Bound native cargo builds and individual WASI tests with cancellable deadlines

DECISIONS.md

@@ -70,6 +70,16 @@ a plugin, or a change that the agent makes to itself.
   that occur only in WASI, for example a missing WASI function or a panic at
   start. The test runner uses the same Wasmtime and the same mounts as the
   core.
+- **Tests never run without a timeout.** `cargo.build` serializes its work
+  folder, so one hung test or build script must not block all later builds.
+  `timeout_secs` defaults to 900 seconds after mutex acquisition, excluding
+  queue time; `test_timeout_secs` defaults to 120 seconds per WASI instance.
+  Both are optional positive integers, never null/unlimited. Test timeouts name
+  the failing test and let other instances finish. Build deadlines and
+  cancellation kill cargo's process group before dropping the mutex; timeout
+  errors retain the phase, elapsed time, and recent output. Source preparation
+  and publication retain serialization while their blocking filesystem work
+  finishes, even if cancelled, to avoid overlapping writes to the work folder.
 - **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