Move broker control traffic to shared-memory rings

[wdcui]

Active broker requests and responses now use paired shared-memory control rings, preserving request-ID multiplexing and bounded host dispatch. Setup transfers and validates an exact-size ring memfd, while the authenticated Unix socket remains for setup, liveness, cancellation, and teardown. Futex-backed waits and 128-byte slots keep the ring efficient and bounded by the protocol's maximum encoded message size.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

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--- failure function_missing: pub fn removed or renamed ---

Description:
A publicly-visible function cannot be imported by its prior path. A `pub use` may have been removed, or the function itself may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/function_missing.ron

Failed in:
  function litebox_broker_host::serve_connection, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/d712088a100c3c7028c4c13e272be0591aeb0e66/litebox_broker_host/src/lib.rs:214

--- failure trait_missing: pub trait removed or renamed ---

Description:
A publicly-visible trait cannot be imported by its prior path. A `pub use` may have been removed, or the trait itself may have been renamed or removed entirely.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#item-remove
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/trait_missing.ron

Failed in:
  trait litebox_broker_protocol::channel::HostControlChannel, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/d712088a100c3c7028c4c13e272be0591aeb0e66/litebox_broker_protocol/src/channel.rs:67

--- failure method_parameter_count_changed: pub method parameter count changed ---

Description:
A publicly-visible method now takes a different number of parameters, not counting the receiver (self) parameter.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#fn-change-arity
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/method_parameter_count_changed.ron

Failed in:
  litebox_broker_transport::unix_socket::UnixStreamLocalControlChannel::activate takes 1 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/d712088a100c3c7028c4c13e272be0591aeb0e66/litebox_broker_transport/src/unix_socket.rs:153, but now takes 2 parameters in /home/runner/work/litebox/litebox/litebox_broker_transport/src/unix_socket.rs:163
  litebox_broker_transport::unix_socket::UnixStreamHostControlChannel::into_active takes 0 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/d712088a100c3c7028c4c13e272be0591aeb0e66/litebox_broker_transport/src/unix_socket.rs:313, but now takes 1 parameters in /home/runner/work/litebox/litebox/litebox_broker_transport/src/unix_socket.rs:359