Update pgrx version management and standardize multi-PG packaging workflow

[ringsaturn]

Summary

• Add scripts/pgrxw.sh to enforce cargo-pgrx version alignment with Cargo.toml and validate pgrx/pgrx-tests consistency
• Pin pgrx and pgrx-tests to =0.17.0 and map pg14 to pg18 features to both pgrx and pgrx-tests
• Refactor Makefile with test-all, test-pg%, package-all, and package-pg% targets, plus standardized artifact naming and smoke checks
• Enhance CI to detect pinned pgrx version, use wrapper-based pgrx commands, generate metadata-rich artifact names, and validate release artifact completeness across PG14 to PG18
• Update docs with the multi-artifact compatibility model and add a release runbook in RELEASING.md
• Update .gitignore to ignore tmp/, docs/, and dist/

Testing

• cargo metadata --no-deps --format-version 1
• ./scripts/pgrxw.sh --help
• make test-pg15
• make package-pg15