audit F35 [debt]: Arc<Mutex> in willow-actor fsm.rs (move into mod tests if test-only)

[intendednull]

File: crates/actor/src/fsm.rs:415
Severity: quality — debt
Obvious? no

transitions: Arc<Mutex<Vec<(Light, Light)>>> lives in fsm.rs. Inspection shows it inside an async fn test (test-only). The audit grep flagged it because the exclusion glob '!**/tests*' does not catch in-file test modules.

Fix: if test-only, move into a #[cfg(test)] mod tests block whose path matches the exclusion to keep audits clean. Saves false-positive triage in every future audit.

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Filed by /general-audit @ b901575 (2026-05-02). master: #513.