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Choose the lowest meaningful test layer and design deterministic test harness ownership. Use for unit, component, adapter-contract, integration, or E2E placement; suite-load or CI-only failures; timeout, retry, sleep, serialization, warmup, timing-workaround, or suite/config-splitting proposals; and browser, Electron, mobile, subprocess, server, port, process, isolation, readiness, diagnostic, or cleanup ownership. Covers cross-platform proof boundaries and integration-suite reachability. Not for test-framework selection, coverage thresholds, general mocking or assertion style, visual verification, host-OS path or quoting mechanics, or Post.Build.Ship gate and receipt policy.