fix: compile join right-side pipelines

[icostan]

Summary

Fixes Dux.join/3 and Dux.asof_join/3 silently discarding pending operations on the right-hand pipeline.

Changes

• Compile the complete right-hand %Dux{} pipeline instead of only its source.
• Use the caller’s database connection when compiling nested pipelines.
• Propagate nested setup SQL through the outer query build.
• Add regression coverage for right-side:
  • filter
  • mutate
  • summarise
  • summarise_with
  • ASOF join operations
  • setup SQL generation

This prevents joins from silently operating on unfiltered or unaggregated right-side data.

Suffix handling is intentionally unchanged and remains follow-up work.

Testing

• mix test test/dux/query_test.exs — 56 tests passed
• Focused query/join/ASOF suites — 132 tests passed
• mix format --check-formatted — passed

The full suite reached 899/900 tests; the unrelated distributed pivot_longer test still fails, while 16
PostgreSQL tests were invalidated by container startup failures. mix compile --warnings-as-errors
remains blocked by three pre-existing warnings.