Closes the 8a follow-up. string.GetHashCode is stable on Mono but randomized
per process on .NET Core, so a headless sim that generates zones would get
different zones on every run.
- Extensions.StableHash folds a string's UTF-8 bytes through CultMath's pcg
from the FNV-1a offset basis. No new CultMath API.
- ZoneGenerator seeds from `Name.StableHash() ^ pcg3d(Position).x`.
- Zone seeds from `Name.StableHash() ?? 1337u`. CultMath's Random maps
seed 0 to its default, and the abs/Convert.ToUInt32 pair that threw on
int.MinValue is gone.
Old runs are discarded, so a different galaxy per seed costs nothing.
Tests: StableHashIsNotProcessRandomized pins "Adrasteia" to 2427892944, the
value an independent Add-Type transcription of CultMath's pcg produced. It
fails with GetHashCode substituted back in.
Builds: headless, 15 tests pass, Unity batchmode compile reports no error CS.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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