Add a shared broker notification ring

[wdcui]

Extend the exact shared control mapping with an independent 256-slot broker-to-local notification ring while keeping request and response capacity unchanged. Make ring capacity direction-specific, expose role-bound endpoints, and pin notification wire size; production notifications remain on the Unix socket until the next migration PR.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

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--- failure enum_variant_added: enum variant added on exhaustive enum ---

Description:
A publicly-visible enum without #[non_exhaustive] has a new variant.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#enum-variant-new
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/enum_variant_added.ron

Failed in:
  variant ControlRingDirection:Notifications in /home/runner/work/litebox/litebox/litebox_broker_transport/src/control_ring.rs:67