ShipEffects: cache SoundLayerGroups globally instead of parsing per-vessel

[bimo1d]

Hi again, follow-up to the debris-skip PR.

After debris are filtered out, the remaining hitch on separations comes from non-debris vessels. Profiler shows ~120ms in ShipEffects.Initialize, of which ~117ms is the SoundLayer parsing loop.

ShipEffectsConfig is global, AudioClips from GameDatabase are immutable, so the SoundLayer objects produced by CreateSoundLayerGroup are identical for every vessel. Parsing them once per vessel is pure overhead.

This PR moves the parsing into a lazy static cache (GetCachedSoundLayerGroups). Each vessel's Initialize copies references from the cache instead of re-parsing. SoundLayer instances are shared across vessels, but the per-vessel SoundLayerGroups dictionary owns it's own list copies to preserve existing per‑vessel isolation.

Thanks!

[JonnyOThan]

Thanks for this; I implemented it a slightly different way but this is a great idea!