libp2p-rendezvous: Unbounded rendezvous DISCOVER cookies enable remote memory exhaustion

### Summary The rendezvous server stores pagination cookies without bounds. An unauthenticated peer can repeatedly issue `DISCOVER` requests and force unbounded memory growth. ### Details Pagination state is stored in: ```rs HashMap<Cookie, HashSet<RegistrationId>> ``` On `Message::Discover`: ``` remote peer → DISCOVER → handle_request → registrations.get(...) → new cookie generated → cookie inserted into Registrations::cookies ``` There is **no upper bound or eviction policy**, so repeated DISCOVER requests grow this map indefinitely. ### PoC A reproduction test and minimal harness will be provided in a private fork in a follow-up comment. ### Impact **Remote state amplification leading to memory exhaustion.** Properties: - etwork reachable - no authentication required - low attack complexity - protocol-compliant traffic Impacts rendezvous nodes exposed to untrusted peers. --- ### Possible Fixes 1. **Global cap + eviction** Bound cookie storage (`MAX_COOKIES_TRACKED`) with FIFO/expiry aware eviction. Tradeoff: attacker can churn cookies and evict legitimate pagination state. 2. **Stateless cookies** Encode pagination state in authenticated cookies instead of storing server-side state. Tradeoff: more complex implementation. 3. **Rate limiting / per-peer quotas** Limit cookie creation per peer. Tradeoff: requires peer tracking.