client(async): narrow stubbed to pub(crate); drop dead message_bus (#574)

* docs(plans): add hide-internal-types tracking plan

Audit + 4-PR breakdown for v3-api-ergonomics.md §3. Item 1: async/sync
stubbed/message_bus asymmetry. Item 2: pub mod proto (zero external
consumers). Item 3: sort messages — re-export user-facing types from
crate root + prelude, demote wire internals to pub(crate). Item 4:
verification checkpoint.

* client(async): narrow stubbed to pub(crate); drop dead message_bus accessor

PR 1 of plans/hide-internal-types.md. Restores sync/async parity —
sync had pub(crate) fn stubbed since #480 with no message_bus()
accessor; async drifted to pub. Both items were gated #[cfg(test)] so
external surface is unchanged.

message_bus() had zero callers anywhere in the workspace (internal
code reads the pub(crate) field directly); deleted per rule 9
(modernize touched module).