Add peer card discovery scaffold

docs/roadmap.md

@@ -101,14 +101,14 @@ geth-to-geth connections without granting trust from discovery alone.
   - Unknown ALPNs are rejected explicitly.
   - Tests cover registration collisions and unknown protocol handling.
 
-- `[ ]` Peer cards.
+- `[x]` Peer cards.
   Acceptance criteria:
   - A peer card contains node ID, agent ID, endpoint candidates, timestamp, and
     signature metadata.
   - Peer cards are stored in `peer_cards`.
   - Invalid or unsigned peer cards do not update trust state.
 
-- `[ ]` Untrusted discovery backend trait.
+- `[x]` Untrusted discovery backend trait.
   Acceptance criteria:
   - Discovery returns candidate peer cards only.
   - No discovery result grants capabilities or trust.