maxRenderableFloatBits

Probed lazily on first access: builds a tiny RGBA32F FBO, then a tiny RGBA16F FBO, and checks glCheckFramebufferStatus. iOS GLES3 on real devices typically supports both; iOS Simulator on Mac silicon (Metal-backed) often supports only RGBA16F. The result is cached so the probe runs at most once per IosKgl instance, after the GL context is current (see WorldWindow.eaglContext).