feat(postgresql): add MERGE support

[taua-almeida]

Summary

• add PostgreSQL MERGE parsing, AST traversal, formatting, analysis, and code-generation support
• infer parameters across source queries, join conditions, WHEN conditions, updates, and inserts
• support RETURNING, including source-before-target star expansion, NOT MATCHED BY SOURCE nullability, and typed merge_action()
• add documentation and end-to-end coverage for database/sql, pgx/v4, and pgx/v5

Fixes #2272

Test plan

• go test ./internal/compiler ./internal/engine/postgresql ./internal/sql/...
• MERGE end-to-end replay tests
• MERGE and UPDATE formatter round-trip tests
• go test ./... -run '^$'
• git diff --check
• make build-endtoend
• full go test ./... using PostgreSQL 16.14 for base and managed-db contexts

MERGE ... RETURNING, merge_action(), and NOT MATCHED BY SOURCE fixtures remain base-only because those features require PostgreSQL 17 or later.