Constant RAIL_NOTE 

pub const RAIL_NOTE: &str = "energy: GPU RAIL ONLY (nvidia-smi power.draw.average, 250 ms cadence). \
     On GB10 the module rail — Grace cores + LPDDR5X — reads N/A and is NOT in this number; on a \
     bandwidth-bound unified-memory decode a dominant share of real energy is outside it. Do not \
     compare to a discrete card's board power.";

2026-09-26: Logged once per run when sampling starts (EnergyMeter::start).