child
Derives a stable child seed from the installed master seed for a labeled sub‑run.
The derivation uses the label hash, the optional index, and a SplitMix64 finalizer with a golden‑ratio increment to decorrelate streams.
Determinism requires that installDeterministic has been called before.
Return
a 64‑bit seed suitable for initializing a new RNG
Parameters
label
semantic label for the child stream (for example "BEST")
index
optional index to obtain additional distinct seeds under the same label