Migrate @mongodb-js/zstd off the prebuild-install install script so it installs under npm 12 (which blocks dependency install scripts by default).

Approach: prebuildify + node-gyp-build. Bundle all-platform prebuilds inside the npm tarball (~3.4 MB total across the 8 current targets) and resolve the right one at require-time. No install script.

Acceptance Criteria:

• Installs and loads under npm 12 with no allowScripts approval.
• CI runs on npm 12 and all builds succeed.