Old shape compared listTmpStageDirs() snapshots before/after the
mid-copy throw. Under scripts/run-tests.cjs --test-concurrency=4,
tests/install-minimal-all-runtimes.test.cjs runs in a parallel
subprocess and also creates gsd-minimal-skills-* dirs in shared
os.tmpdir(). The parallel process's create/remove activity between
this test's two snapshots caused deterministic failure when timing
aligned -- presented as 'flaky' but is a real race.
CI failure data (PR #2993 run 25238555786):
expected (before): ['gsd-minimal-skills-km1O1O']
actual (after): []
Both processes behaved correctly in isolation. The test was wrong:
it observed a shared filesystem state across processes.
Fix: stub fs.mkdtempSync inside this test to record THIS call's
stage dir path. After the throw, assert fs.existsSync(stagedDir)
=== false. Direct observation of the function's own behavior; no
global tmpdir scan; no parallel-process interference.
Closes#3008
