[SEC-V-06] `TopicAnnounce.topics: Vec<String>` has no element-count cap; enables relay CPU amplification + topic-slot exhaustion

[intendednull]

Commit: 2f26d91 · Finding: SEC-V-06

Problem

The relay's topic subscription listener at crates/relay/src/lib.rs:371-423 caps the total subscribed-topic set at MAX_TOPICS = 10_000 but iterates topics per-message unbounded. topics: Vec<String> is declared at crates/common/src/wire.rs:76 with no per-message cap.

256 KB of 2-byte topic strings ≈ 128,000 topic entries to process in one message — each calls topic_str_is_valid (O(n) on length) and willow_network::topic_id (a blake3 hash).

Additionally, topics are never removed once added, so a peer can pump up to 10,000 attacker-controlled topics into the slot table and deny legitimate clients' announces thereafter.

Fix

• Cap topics.len() per announce (e.g. 64) before iterating; reject messages exceeding the cap.
• Enforce a per-signer subscription cap (e.g. 100 topics per announcing peer) with LRU eviction so one peer cannot monopolise all 10,000 slots.
• Upgrade the existing warned_full log from once-per-session to rate-limited (1 per minute), so operators see the wall.