PIPE-3269: pre-compute importdata flag summaries
- Pre-compute baseline flagdata summaries across imported MeasurementSets during ImportData.prepare() using parallel_map, and store them directly on ImportDataResults.
- Update ImportDataQAHandler to passively consume the cached flag summaries and model data columns rather than issuing redundant disk queries against vis.ms. This decouples background QA rendering from subsequent in-place MS mutations, preventing table lock contention.
