Archive specta-rs/builder (buld) in smeltr

The builder repository is being retired and its work kept here so nothing is
lost. This branch's tree is byte-identical to specta-rs/builder@4b49854
("make things more stable"); it is a frozen snapshot, not a port.

The original TODO issue (specta-rs/builder#1) is preserved in this PR's
description because deleting the repository deletes its issues.

This branch is deliberately NOT meant to be merged: it predates smeltr's
rewrite and has unrelated history, so merging it would replace smeltr's tree.

Parents:
  - smeltr main (337053f) so the branch can be compared in a pull request.
  - builder main (4b49854) so builder's full commit history stays reachable.