cargo: run host unit tests before builds

PROTOCOL.md

@@ -231,9 +231,19 @@ folder again, as at start.
 {"type": "build", "source": "/actor/agent/public/drafts/notes"}
 ```
 
-The reply is `{"type": "built", "wasm": "/actor/cargo/public/builds/<n>/<name>.wasm", "built_by": "cargo", ...}`
-or `{"type": "build_failed", "errors": "..."}`. Pass `wasm`, `source`, and
-`built_by` on to `spawn` or `upgrade`.
+The build request may include `"skip_tests": true` to skip Rust unit tests.
+By default, `cargo` first runs `cargo test` for the host target (not WASI),
+so pure-logic unit tests in a `wasm32-wasip2` actor crate can run without a
+WASI test runner. If tests fail or cannot compile, the build fails before the
+WASI build. The reply carries `tests: {"passed": <count>, "failed": <count>,
+"output": "<cargo test stdout and stderr>"}`; counts sum Cargo's test-result
+summaries, and a test command/compilation failure without a summary counts as
+one failure. With `skip_tests: true`, counts are zero and output says tests
+were skipped.
+
+The reply is `{"type": "built", "wasm": "/actor/cargo/public/builds/<n>/<name>.wasm", "built_by": "cargo", "tests": {...}, ...}`
+or `{"type": "build_failed", "errors": "...", "tests": {...}}`. Pass
+`wasm`, `source`, and `built_by` on to `spawn` or `upgrade`.
 
 A Rust actor depends on the SDK with `sausage-actor = "0.1"`. `cargo` points
 that dependency at the SDK of the host, which it publishes at