parkBenignSightlineCube

Parks a benign cube on unit 5 and invalidates the sightline bind cache. Called by DrawableSightline whenever it clobbers unit 5 for feedback safety (a texture bound for sampling can't also be a render target). Without the lastSightlineTextureBind reset a later receiver holding the same SightlineState would cache-hit and sample this benign cube instead of the real depth cube - safe today only by drawable ordering. The parked cube is compare-mode (nullShadowDepthCubeTexture) under hardware samplers, matching the receiver path so WebGL2's draw-time sampler-vs-format validation stays consistent.