Function ssm_tail_ckpt_enabled 

pub fn ssm_tail_ckpt_enabled() -> bool

2026-09-25: Tail-checkpoint chunk split, on only with METRALE_SSM_TAIL_CKPT=1 (see ssm_tail_boundary). Resolved on first call and cached.

Off by default: an A/B measured 2026-07-10 (174 samples per arm) found it perf-neutral, because the extra chunk split (median 868 ms) cancelled the SSM replay it saved (about 1374 ms).