instancingGatherPlacementKey

Integer placement zoom for buildInstancedWindBufferDualPhase / buildViewportGridPoints cache keys.

Instanced wind renderers must use this in their gather key — not densityKeyForMapZoom (64 fractional bins per zoom level). Matching stablePlacementZoomInt: small fractional zoom changes during animation were invalidating gather while geometry stayed identical, forcing expensive full CPU rebuilds every frame.