Preload gateway signature verification keys

[zjma]

Summary

• replace request-time Aptos signature-key resolution with an in-memory gateway registry
• single-flight concurrent holder-key preloads across VSS dealer clients sharing a gateway
• preload all session holder keys before enabling VSS share serving, so ShareRequest handling performs no chain or DB reads
• update auditor documentation for the gateway verification lifecycle

Validation

• cargo test -p node-msg-gateway -p vss-dealer -p vss-recipient -p dkg-worker -p dkr-src
• pnpm --dir scenarios test-vss-protocol
• rustfmt --edition 2021 --check worker-components/node-msg-gateway/src/lib.rs worker-components/vss-dealer/src/lib.rs
• git diff --check