Loads + composites an ImageryStack for a terrain tile's rectangle into one draped
texture, mirroring loadImageryWithFallback's return shape (so the terrain path is
unchanged). Each layer's mosaic is fetched best-effort; failed layers are dropped, and the
first that loads becomes the base block. A single identity layer skips the composite pass.
Returns null only when every layer failed (→ caller falls back to the elevation ramp).
Loads + composites an ImageryStack for a terrain tile's rectangle into one draped texture, mirroring loadImageryWithFallback's return shape (so the terrain path is unchanged). Each layer's mosaic is fetched best-effort; failed layers are dropped, and the first that loads becomes the base block. A single identity layer skips the composite pass. Returns null only when every layer failed (→ caller falls back to the elevation ramp).