Fix macOS source installs

[Pino de Candia (pinodeca)]

Summary

• derive the PostgreSQL extension library suffix for Linux and macOS source installs
• exercise .so and .dylib install/uninstall contracts for PostgreSQL 17 and 18
• add a native macOS PG17 CI smoke test that packages, installs, preloads, and creates the extension
• update developer helpers and source-platform documentation for .dylib

Validation

• ./scripts/test-make-install.sh
• bash -n scripts/test-make-install.sh scripts/pg-start.sh scripts/test-coverage.sh
• cargo fmt --package pg_durable -- --check
• workflow YAML parse and VS Code diagnostics
• git diff --check

Fixes #348