#3639 bundleExtension 'bundles on different caches do not wait for each other' test fails under CPU load

src/domain/models/bundle_test.ts 'bundleExtension: bundles on different caches do not wait for each other' (line ~1416, added in #3174 for swamp-club#3556) failed in verify-build on 2026-10-19 with state.peak 1 instead of 2, while the host load average was about 87 (swamp-uat uat:cli running alongside). It passed 3/3 when re-run alone. The fake bundler only overlaps the two runs across a setTimeout(0); before each handler runs, bundleExtension does file I/O (makeTempFile, readTextFile), so under load the first run can finish before the second starts and the peak is 1 even though neither waited for the other. Seen on commit 667825b9 (swamp-club#3613 branch), which adds no await on this path. Expected: the test proves the two runs were not serialised without depending on scheduling, for example by holding the first handler on a promise until the second handler has started.