Function decode_rollback_ring_slots_with 

pub fn decode_rollback_ring_slots_with(
    num_ssm_layers: usize,
    use_speculative: bool,
    published: Option<usize>,
    ring_override: Option<&str>,
    watchdogs_disabled: bool,
) -> DecodeRingDecision

Pure core of decode_rollback_ring_slots (env-free, unit-testable).

Precedence, highest first:

1. no SSM layers — there is no recurrent state to snapshot;
2. published — --ssm-decode-ring-slots N, or the depth preflight’s auto-fit chose. It outranks the env override AND the implicit skips for the same reason ATLAS_SSM_DECODE_RING=1 always has: an operator (or a reserve that has already been sized against free memory) asking for a specific depth must get that depth on BOTH sides, or the two diverge;
3. ATLAS_SSM_DECODE_RING=1|0 — the legacy spelling of “8” and “0”;
4. the implicit skips (speculative decode, watchdogs off);
5. the default depth.