ES|QL Subqueries in FROM: Combine independently processed data sources in a single query (GA)

## Value Proposition

Analysts and engineers regularly need to combine data from multiple indices or differently-processed views of the same data within a single investigation — merging raw events with pre-aggregated summaries, combining filtered slices from different data streams, or unioning a live index with a lookup-enriched subset. Subqueries in FROM let users define independent processing pipelines inline and combine their results into a single row stream within one query, eliminating the need for multiple sequential queries with client-side stitching.

## Expected Outcome

Subqueries in FROM shipped in Tech Preview in Elasticsearch 9.4, where users can define inline processing branches and combine results.

For GA, we are bringing subqueries to production readiness:

- **Broader source command support:** ROW and TS commands work inside subquery branches, alongside FROM
- **Data Federation compatibility:** subqueries can query federated data sources
- **Bug fixes and correctness:** resolved type conflicts, interactions with INLINE STATS and LOOKUP JOIN, and schema consistency across branches
- **Performance:** column pruning optimizations for subquery branches
- **CCS compatibility:** correct cluster status reporting for cross-cluster subqueries
- **ENRICH scope:** fix ENRICH index resolution within subquery context