launchIsolatedApp() binds a non-reserved port (reuses the 4040/FUSION_RESERVED_PORTS
guards), serves a freshly-built bundle, runs against a fresh/empty disposable DB
under a run-unique tmpdir with a scrubbed env (no inherited credentials), and tears
down unconditionally on crash/timeout/abort. Process-spawn and bundle-build are
injected seams; 18 tests characterize the R13 isolation contracts. Real end-to-end
launch wires the default spawner behind the seam in a heavier lane.
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