Add shared-memory broker pipe data path

[wdcui]

Summary

• add transport-neutral shared-memory attachments and offset/length pipe operations
• transfer one sealed memfd per pipe over Unix sockets with separate read/write staging regions
• keep BrokerCore authoritative through synchronous control requests and preserve inline fallback
• allow only the Linux-userland syscalls needed to receive and validate shared-memory descriptors

Testing

• cargo fmt --check
• cargo test -p litebox_broker_protocol -p litebox_broker_local -p litebox_broker_host -p litebox_broker_transport
• cargo test -p litebox pipes
• cargo clippy -p litebox_broker_protocol -p litebox_broker_local -p litebox_broker_host -p litebox_broker_transport -p litebox_platform_linux_userland -p litebox_runner_linux_userland -p litebox --all-targets --all-features -- -D warnings
• cargo test -p litebox_runner_linux_userland --test run test_runner_broker_integration_with_rewriter -- --exact

[wdcui]

Superseded by the focused stack: #1049 (protocol), #1048 (broker host), #1051 (broker local), and #1050 (Unix transport).

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- 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/v0.49.0/src/lints/constructible_struct_adds_field.ron

Failed in:
  field CreatePipeResponse.shared_memory in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/pipe.rs:37

--- 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 PipeResponse:ReadShared in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/message.rs:121
  variant PipeResponse:WriteShared in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/message.rs:125
  variant PipeRequest:ReadShared in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/message.rs:80
  variant PipeRequest:WriteShared in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/message.rs:84

--- failure trait_associated_type_added: non-sealed public trait added associated type without default value ---

Description:
A non-sealed trait has gained an associated type without a default value, which breaks downstream implementations of the trait
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#trait-new-item-no-default
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/trait_associated_type_added.ron

Failed in:
  trait associated type litebox_broker_protocol::channel::HostControlChannel::SharedMemory in file /home/runner/work/litebox/litebox/litebox_broker_protocol/src/channel.rs:81
  trait associated type litebox_broker_protocol::channel::LocalControlChannel::SharedMemory in file /home/runner/work/litebox/litebox/litebox_broker_protocol/src/channel.rs:51

--- failure trait_method_parameter_count_changed: pub trait method parameter count changed ---

Description:
A trait method now takes a different number of parameters.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#trait-item-signature
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/trait_method_parameter_count_changed.ron

Failed in:
  HostControlChannel::send_response now takes 2 instead of 1 parameters, in file /home/runner/work/litebox/litebox/litebox_broker_protocol/src/channel.rs:112