philihp / pcg / 26189404174

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Add sfc32 as another variant (#340)

sfc32 (Chris Doty-Humphrey, PractRand) is a 128-bit-state chaotic PRNG
built from add, xor, shift, and rotate. It complements pcg32 (high
quality, large state, jump-ahead) and mulberry32 (tiny state, weaker
quality): sfc32 is fast and passes PractRand, but isn't reversible, so
negative deltas throw.

State packing: a=state.hi, b=state.lo, c=streamId.hi, counter=streamId.lo.
Seeding uses a=b=c=seed, counter=1, with 15 warmup rounds.

The dispatch contract widens from (pcg, delta) -> Uint64 to (pcg, delta)
-> PCGState so variants can evolve fields beyond .state — sfc32 needs to
update all four registers.

Co-authored-by: Claude <noreply@anthropic.com>

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