Cache sits between override resolution and platform_probe; override
paths (test + env-var) bypass it entirely so they always take effect
immediately. OnceLock<Mutex<Option<CachedState>>> initialised lazily on
first probe. Two test-only helpers (force_clear_cache, force_set_cache)
expose the cache slot for unit tests. Mutex import ungated — now used in
production cache code as well as test override.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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