docs(chapter): add U4 CodeQ IO chapter covering gRPC, REST, Pebble, raft (#628)

Walks the three I/O layers — gRPC streaming surface, REST mirror,
group-commit persistence engine, and optional raft replication — in
prose, with a closing section on how they compose under load.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: osvaldoandrade