fix(test): anchor the knob-lab scaffold under CARGO_TARGET_TMPDIR

built_scale_project() scaffolded ~66 MB (built node crate) into a
pid-suffixed tempdir, so its wipe-before-scaffold never hit a previous
run's copy: one leaked directory per test-binary run. 159 of those
(~10 GB) filled the host's 16 GB /tmp tmpfs to 100%, breaking every
std::env::temp_dir() consumer. A fixed name under CARGO_TARGET_TMPDIR
(real disk, reclaimed by cargo clean) makes the existing wipe reclaim
the previous run — steady state is exactly one directory. Trade-off:
two concurrent cargo-test invocations over the same checkout collide
on the fixed name, the documented pre-existing caveat class for this
suite (#223).

closes #257

crates/aura-cli/tests/project_sweep_campaign.rs

@@ -63,11 +63,15 @@ fn aura_in(dir: &Path, args: &[&str]) -> Output {
 /// `blueprints/scaled_open.json`. Returns the project dir. `OnceLock` so the
 /// expensive scaffold+build happens once per test binary; the node crate's
 /// path-dep resolves into this checkout (the baked engine root), like
-/// `project_new.rs`.
+/// `project_new.rs`. The base is a FIXED name under `CARGO_TARGET_TMPDIR`
+/// (real disk, reclaimed by `cargo clean`), not a pid-suffixed tempdir: the
+/// wipe below must hit the PREVIOUS run's copy, else every test-binary run
+/// leaks the ~66 MB built scaffold — 159 of those filled the 16 GB /tmp
+/// tmpfs to 100 % (#257).
 fn built_scale_project() -> &'static PathBuf {
     static BUILT: OnceLock<PathBuf> = OnceLock::new();
     BUILT.get_or_init(|| {
-        let base = std::env::temp_dir().join(format!("aura-235-{}", std::process::id()));
+        let base = Path::new(env!("CARGO_TARGET_TMPDIR")).join("aura-235-knob-lab");
         let _ = std::fs::remove_dir_all(&base);
         std::fs::create_dir_all(&base).expect("create scaffold base");