feat: add connect forwarding recency filter

[sanity]

Do not review - this will be replaced in a subsequent PR

Problem

Connect forwarding retried the same neighbors without regard to recent attempts, leading to saturation and low degree in the soak; routing_candidates wasn’t exposed for filtering.

This change

• Add a per-op recency map to connect forwarding; prefer least recently forwarded neighbors, missing recency wins, then use router selection.
• Track recency on forwards and clear on acceptances; add routing_candidates helper to the connection manager to support the filter.

Testing

• cargo check -p freenet (local).