[RyanKung]

Summary

Closes #1222 and #1223.

Issue #1222

1. send_connection_offer / Swarm::offer_connection pending slot leak:
   • Routes sender.originate(...) and self.mark_offer_sent(...) errors through SwarmTransport::classify_step_failure so any failure while attempt still owns the unadmitted slot immediately abandons the pending slot and closes the raw WebRTC connection.
2. Candidate dial backoff misaccounting:
   • Tracks was_offering: bool on RetiredPendingConnection and ExpiredUnadmittedPeer at the atomic moment the Pending record is removed under the lifecycle lock.
   • Records record_candidate_dial_failure only when a retired/expired attempt owned an outbound Handshake::Offering slot, and exempts local capacity refusals (PendingConnectionCapacityExceeded, ConnectionCapacityExceeded) from peer backoff penalties.
3. Glare finger lease rebind (yield_pending_in):
   • Adds SwarmTransport::rebind_pending_finger_updates(from, to) and updates yield_pending_in to rebind any queued finger convergence proof from the yielding outbound offering attempt to the replacing inbound answering attempt instead of cancelling the active finger lookup.

Issue #1223

1. Preserve onion hop decryption across in-process delegation renewal:
   • Exposes decryption_keys_at(&self, now_ms: u128) -> Vec<DelegateeKey> on extension::ext::registry::{Core, Scope} backed by SessionKeyRing::decryption_keys_at.
   • Updates OnionCircuitShell::hop, circuit::hop::hop, Charged::peel, OnionCell::peel, and peel_in_place to accept keys: &[DelegateeKey] and try current followed by retained unexpired pre-renewal keys when verifying the per-hop Sphinx HMAC.
2. Scope DelegationLessee::encode within_margin check to delegations.hop:
   • Enforces within_margin(delegations.hop.expires_at_ms(), now_ms) only on delegations.hop, allowing delegations.origin as long as expires_at_ms() > now_ms and falling back to DelegationRef::inline(delegation) inside the final LEASE_MARGIN_MS.
3. Enforce delegator signature & monotone horizon in SwarmTransport::set_delegatee_key and serialize renewal under session_keys write lock.

Test Plan

• cargo test -p rings-core --features dummy --lib swarm::session_link
• cargo test -p rings-core --features dummy --lib swarm::transport::tests::test_finger
• cargo test -p rings-core --features dummy --lib swarm::transport::tests::test_lifecycle
• cargo test -p rings-node --lib onion::
• cargo test -p rings-node --lib horizon

[RyanKung]

Reviewed at 38ef5e4efc98388dc1ad06728ea30c7a0f755919. I found no confirmed actionable defect introduced by this PR within the reviewed scope.

• Handshake ownership and backoff: traced offer-origination and mark-sent failures through retirement. Outbound ownership is captured under the lifecycle lock; inbound Answering, superseded attempts, and capacity refusal do not incorrectly charge outbound dial backoff.
• Glare and finger updates: checked rebinding the pending finger proof to the replacement generation. Old-generation cancellation cannot remove the newly keyed proof.
• Decryption and authority: followed active and retained keys from SessionKeyRing into Sphinx peeling. Invalid group elements fail before key trial, and header mutation follows a matching HMAC. Retained keys are expiry-filtered; the setter checks delegation authority and the signing horizon.
• Renewal and compatibility: checked validation, persistence, and swarm-key installation within the ring write-lock transaction, plus origin-versus-hop lease-margin behavior. Wire encoding is unchanged; retained-key trial cost is linear in the unexpired key count.

Local validation: cargo +nightly-2026-07-02 fmt --all -- --check passed. The added lifecycle, glare, setter, retained-key, and lease-margin tests were inspected, but Rust tests were not rebuilt or rerun locally at this head. Separately, the inspected current-head GitHub CI report showed 19 successful checks and one skipped check.

Limits: no local browser-runtime execution or retained-key performance benchmark. The overlapping registry changes in #1248 were not jointly tested with this PR.