Add rustdoc to public protocol types

[bhekanik]

Improve discoverability in crates/protocol/src/lib.rs by documenting the main public IPC types.

Acceptance criteria:

• Add rustdoc for the top-level protocol module and its primary public exports
• Focus on Request, Response, and IPC framing concepts
• Avoid restating obvious field names; document intent and boundaries instead
• RUSTDOCFLAGS=-Dwarnings cargo doc --workspace --no-deps stays green