Address Copilot review: the mapping table was
defined but never referenced. Now `mk_maddr()`
resolves `proto_key` -> maddr protocol name via
the table and rejects unknown keys upfront.
Also add missing `Path` import to the `multiaddr`
usage snippet.
Review: PR #429 (Copilot)
https://github.com/goodboy/tractor/pull/429#pullrequestreview-4010456884
(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code
Gud Boi