fix(peer): register an accepted relayed circuit in the connected-peer pool

[MichaelTaylor3d]

WIP — dig_ecosystem#870 / #1871 CONNECT leg. Consumes the dig-gossip relayed-inbound adoption API from serve_accepted_relay_conn, raises the relayed-accept failure log to warn, and adds a connection-closed event.

Refs DIG-Network/dig_ecosystem#870

[MichaelTaylor3d]

Closing: this branch is empty.

Its single commit b3fb152 (chore: open #870 connect-leg lane) is a §1.8 push-early lane opener — changedFiles: 0, additions: 0, deletions: 0. git diff <merge-base>...origin/feat/870-relayed-responder-pool produces nothing. The implementer never returned, so 100% of the work remains. Check version increment correctly FAILS (nothing is bumped); the other nine checks pass only because the branch is byte-identical to a 16-commit-old main.

The gap it names is real and still open — crates/dig-node-core/src/peer.rs:2222 serve_accepted_relay_conn builds relayed_caller_contact and calls serve_peer_session_from but never adopts the peer into the pool; none of the four adopt_nat_connection call sites (peer.rs:3455, :3467, :3493, seams/dig_peer/pex.rs:652) is on the relayed-accept path. The dig-gossip API it needs exists at dig-gossip/src/service/gossip_handle.rs:1241.

But dig_ecosystem#870 carries status:deferred, so an empty PR against it is noise. #870 stays open with the diagnosis above. When it is picked up it should be re-cut from current main rather than resumed from a 16-commit-stale empty branch.