Optimize network hot paths on client and server

[RZDESIGN]

Summary

• reduce callback and allocation overhead in client network packet hot paths
• replace map/forEach/flatMap usage in packet serialization and deserialization hot loops
• tighten chunk update and chunk worker iteration paths

Testing

• git diff --check
• TypeScript transpile syntax check for edited files
• client build still blocked by pre-existing unrelated TypeScript errors in the repo