revert: undo Joiner enum (#2158) to unblock PR stack #2167-#2175

[sanity]

Summary

Reverts #2158 ("fix: use Joiner enum to properly handle NAT peer address discovery")

Why

PR #2158 was merged to fix NAT peer address discovery, but it conflicts with the pending PR stack (#2167 → #2169 → #2171 → #2172 → #2174 → #2175) which takes a more comprehensive approach to the same problem.

The PR stack:

• Implements PeerAddr enum (Unknown/Known) - similar concept but integrated with broader refactoring
• Changes routing to use SocketAddr directly instead of relying on embedded addresses
• Removes from/sender fields from wire protocol messages
• Is awaiting human review

By reverting #2158, the PR stack will merge cleanly and CI can run on those PRs. The PR stack's approach supersedes and improves upon #2158's Joiner enum.

Impact

• Removes Joiner enum from connect.rs
• NAT peer discovery will temporarily regress until the PR stack is merged
• The PR stack (refactor(routing): add upstream_addr for connection-based routing #2167-refactor: remove redundant inline timeout check from connect op #2175) should be merged promptly after this

[AI-assisted - Claude]