Constant TERMINAL_RETENTION_CAP 

const TERMINAL_RETENTION_CAP: usize = 128;

Retention cap for TERMINAL sagas (completed / failed / failed_compensation). When exceeded, the oldest terminal sagas are evicted. In-flight sagas are never evicted — mirroring the old vacuum’s “never vacuum running/compensating” rule. 128 comfortably exceeds anything a session produces (each window close = 1 saga, each pool promote = 1 saga) while bounding memory at a few hundred KB worst case.