4D bilateral filter now measures intensity variance across all timepoints in the mask and skips non-finite neighbours, eliminating spurious NaNs on constant or noisy inputs.
Added regression tests covering the zero-window identity and constant-volume stability for the parallel 4D bilateral filter backend.
New meta_info() helper returns a normalized list of basic header metadata from a filename or FileMetaInfo (dim, spacing, origin, trans, path, etc.), making 3D/4D image introspection simpler for new users.