Without a lock, env_var_bypasses_clamps (set_var + remove_var) and
matches_existing_clamp_when_no_clamps_apply (remove_var) race under
cargo's parallel test runner. Task 2.2 will add another remove_var
call, compounding the race.
Adds a #[cfg(test)] static THREAD_ENV_LOCK and a closure-style
with_thread_env_lock helper, mirroring power::with_override.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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