ddx-pg: Postgres integration via pgrx

[alxmrs]

Needs array/XQL support first (the long/tidy N-d-array data model), then a planner-hook native path; until then the universal source-to-source rewrite with PostgreSqlDialect applies unchanged. (design.md §1.1, §3.4, §8 Future)

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_{Curated from the project roadmap; see docs/design.md §8 for the milestone plan.}