truapi-server: golden_rust_emit test races on target/doc/truapi.json and silently passes without nightly #99

Found during review of #96. rust/crates/truapi-codegen/tests/golden_rust_emit.rs writes/reads the same workspace target/doc/truapi.json path concurrently when cargo test parallelizes, and silently returns success when cargo +nightly rustdoc is unavailable (eprintln! + return). On a no-nightly CI the test is vacuously green. Fix: serialize the two tests (per-test tempdir) and either gate them behind a nightly/expensive feature or fail-loud when nightly is missing.