Entity's constructor now takes a MessagePack round-trip copy of the settings
it is given, so no entity can alias GameplaySettings.DefaultEntitySettings.
That makes ActionGameManager.NewEntitySettings and EntitySerializer.Unpack's
null-coalescing copy redundant; both are deleted, and TradeMenu buys a ship
straight off the template (Entity's ctor copies it).
EntitiesDoNotShareDefaultEntitySettings pins the invariant: writing
ShutdownPerformance on one ship must not move the default for a sibling or
the template (settings-globals-cut.md, Cut 0).
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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