Wire broker readiness notifications

[wdcui]

Summary

• add paired broker host serving over control and notification channels, with no control-only serving path
• add BrokerNotifications for local notification receipt
• wire broker-userland and the Linux userland runner through paired --broker-control-socket / --broker-notification-socket Unix endpoints
• keep event mutation readiness in the control response and avoid duplicate notifications for those control-originated changes

Notes

• keeps BrokerCore/BrokerSession transport-neutral
• keeps blocking broker-backed eventfd dispatch for a later PR; the runner opens and receives the notification stream so broker-originated wakeups have a real local endpoint
• current event readiness changes all originate from control requests, so there is no runtime readiness notification emission yet

Validation

• cargo test -p litebox_broker_host
• cargo test -p litebox_broker_userland -p litebox_runner_linux_userland --test run test_runner_broker_integration_with_rewriter
• cargo test -p litebox_broker_userland
• cargo clippy -p litebox_broker_userland -p litebox_runner_linux_userland --all-targets --all-features -- -D warnings

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure function_parameter_count_changed: pub fn parameter count changed ---

Description:
A publicly-visible function now takes a different number of parameters.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#fn-change-arity
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.48.0/src/lints/function_parameter_count_changed.ron

Failed in:
  litebox_broker_host::serve_connection now takes 3 parameters instead of 2, in /home/runner/work/litebox/litebox/litebox_broker_host/src/lib.rs:38

--- failure function_requires_different_generic_type_params: function now requires a different number of generic type parameters ---

Description:
A function now requires a different number of generic type parameters than it used to. Uses of this function that supplied the previous number of generic types (e.g. via turbofish syntax) will be broken.
        ref: https://doc.rust-lang.org/reference/items/generics.html
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.48.0/src/lints/function_requires_different_generic_type_params.ron

Failed in:
  function serve_connection (1 -> 3 generic types) in /home/runner/work/litebox/litebox/litebox_broker_host/src/lib.rs:38

--- failure constructible_struct_adds_field: externally-constructible struct adds field ---

Description:
A pub struct constructible with a struct literal has a new pub field. Existing struct literals must be updated to include the new field.
        ref: https://doc.rust-lang.org/reference/expressions/struct-expr.html
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.48.0/src/lints/constructible_struct_adds_field.ron

Failed in:
  field CliArgs.broker_control_socket in /home/runner/work/litebox/litebox/litebox_runner_linux_userland/src/lib.rs:91
  field CliArgs.broker_notification_socket in /home/runner/work/litebox/litebox/litebox_runner_linux_userland/src/lib.rs:101

--- failure struct_pub_field_missing: pub struct's pub field removed or renamed ---

Description:
A publicly-visible struct has at least one public field that is no longer available under its prior name. It 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/v0.48.0/src/lints/struct_pub_field_missing.ron

Failed in:
  field broker_socket of struct CliArgs, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/b951fc08b4c905beb3bb5d2f5a5e0248363ac9ff/litebox_runner_linux_userland/src/lib.rs:91