PIPE-3250: Implement on-demand multi-SPW selfcal extraction and SubMS applycal integration
- In selfcal._split_scaltargets, extract on-demand channel-averaged multi-SPW working MeasurementSets from calibrated parent EB MSes (DataType.REGCAL_CONTLINE_ALL on CORRECTED_DATA) into sc_workdir with reindex=False.
- Restore multi-SPW gaincal solving (combine='spw') across all project science SPWs, eliminating per-SPW SNR degradation.
- Index child SubMS slices in IntervalCalState.create_from_context, integrating SubMS slices into CalLibrary shape, intent, field, and interval trees.
- In selfcal._apply_scal, apply calibration tables directly to matching physical SubMS slices, populating CORRECTED_DATA across continuum and line slices.
- In SubMS.set_data_column, accept optional arguments and propagate data column registration to parent_ms.
- In SelfcalMosaicRenderer, support string keys for mosaic subfield lookups when deserialized from JSON.
- Add unit test test_callibrary_interval_calstate_subms verifying SubMS indexing in IntervalCalState.
