GHSA-443g-gwgp-49x4: zebrad vulnerable to getblocks/getheaders locator CPU amplification via uncapped vector length

The read_getblocks and read_getheaders codec paths accepted block locator vectors up to approximately 65,535 entries (the generic TrustedPreallocate ceiling derived from MAX_PROTOCOL_MESSAGE_LEN), rather than the protocol-specification limit of 101 entries (matching zcashd’s MAX_LOCATOR_SZ). Each entry in the locator vector triggers a per-hash chain lookup (HashMap::contains_key + RocksDB::contains_hash) in find_chain_intersection on a tokio blocking-pool thread.

A single maximally-sized getblocks message occupies one blocking-pool thread for approximately 10–65ms. Under sustained load from multiple peers, this can degrade state-read performance for block validation, RPC, and mempool lookups.