Add calc = 'rgb' option to ngr_spk_stac_calc()

[NewGraphEnvironment]

Add support for calc = "rgb" in ngr_spk_stac_calc() to stack three bands into an RGB composite.

This allows building RGB composites from native resolution bands (e.g., Sentinel-2 B04, B03, B02 at 10m) rather than using pre-rendered visual assets.

Usage:

ngr_spk_stac_calc(
  feature,
  asset_a = "B04",
  asset_b = "B03",
  asset_c = "B02",
  calc = "rgb",
  aoi = aoi
)

Related to #24