feat(pg): add completion scope context

[rebelice]

Summary

• add PostgreSQL CollectCompletion API returning parser candidates, visible scope, local/outer references, and CTE definitions
• expose top-level pg wrapper and syntax-only range reference types for downstream completion clients
• cover incomplete JOIN scope, lateral/correlated subqueries, recursive CTEs, multi-statement cursor isolation, and alias variants

Test Plan

• go test -count=1 ./pg/...