Function check_legal_flash 

pub fn check_legal_flash(
    t: &FlashTimings,
    plan: &[TimedCmd],
) -> Result<(), String>

The INDEPENDENT legality checker — a total function over the plan, sharing no code with the scheduler. Verifies per-die serialization (a die runs one op at a time, so a read is never issued while its die is mid-erase — the collision the whole design avoids) and per-channel data-bus disjointness. A die’s ops arrive in issue order, so a running watermark is exact; a read’s data-out is DELAYED by tR (sense first), so its bus window need not align with issue order — the channel is checked by interval disjointness, not a watermark. First violation as Err(description).