[ARCH-07] `crates/client/src/lib.rs` is 1,941 LOC with 18 `pub mod` entries — unbounded public surface

[intendednull]

Commit: 2f26d91 · Finding: ARCH-07

Problem

crates/client/src/lib.rs:1-1941 is the single largest file in the crate, declaring 18 pub mod entries plus several pub use shortcuts. ClientHandle has a small intended public API (views(), mutations()), but the enforced public surface includes mentions, mutations, state, state_actors, presence, views, etc.

Callers (web, agent) can — and do — pick from any of them, making "what's public" impossible to reason about.

Fix

• Demote most pub mod to pub(crate) mod or plain mod.
• Re-export the narrow surface callers need: ClientHandle, ClientConfig, ClientError, ClientEvent, ClientView*, ClientMutations, trust types.
• Split lib.rs into lib.rs (~50 lines of re-exports) + handle.rs (ClientHandle / ClientConfig) + error.rs + event_receiver.rs.

Obvious? No — broad API changes.