Make broker pipes shared-memory only

[wdcui]

Summary

• make shared memory mandatory for broker pipes and replace inline payloads with bounded staging offsets, lengths, and byte counts
• scope descriptor-bearing responses to pipe creation through pipe-specific channel traits; successful creation carries shared memory and failures do not
• transfer the sealed memfd with SCM_RIGHTS in the same sendmsg as the serialized create response body while leaving ordinary responses on normal framed I/O
• keep BrokerCore authoritative while host and local adapters stage data through separate read/write regions

Stack

1. Add broker shared-memory primitives #1049 — reusable shared-memory primitives
2. This PR — shared-memory-only broker pipes

Validation

• cargo test -p litebox_broker_transport --all-features
• cargo test -p litebox_broker_protocol
• cargo test -p litebox_broker_local
• cargo test -p litebox_broker_host
• cargo test -p litebox pipes::tests
• cargo test -p litebox event::counter::tests
• cargo test -p litebox_runner_linux_userland --test run test_runner_broker_integration_with_rewriter -- --exact
• strict clippy for affected broker, LiteBox, platform, and runner crates
• no-std and transport feature checks
• cargo test -p dev_tests ratchet_maybe_uninit
• GPT-5.6 Sol static review: no significant issues found

[github-actions]

ℹ️ Note: This semver check was run against the wdcui/ulitebox/broker-pipe-shared-protocol branch, not main or ulitebox.

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure auto_trait_impl_removed: auto trait no longer implemented ---

Description:
A public type has stopped implementing one or more auto traits. This can break downstream code that depends on the traits being implemented.
        ref: https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/auto_trait_impl_removed.ron

Failed in:
  type BrokerLocal is no longer UnwindSafe, in /home/runner/work/litebox/litebox/litebox_broker_local/src/lib.rs:39
  type BrokerLocal is no longer RefUnwindSafe, in /home/runner/work/litebox/litebox/litebox_broker_local/src/lib.rs:39

--- failure inherent_method_missing: pub method removed or renamed ---

Description:
A publicly-visible method or associated fn 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/v0.49.0/src/lints/inherent_method_missing.ron

Failed in:
  BrokerLocal::request, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-wdcui_ulitebox_broker_pipe_shared_protocol/b375c5aa88bef099f1075894100a0735d13f75df/litebox_broker_local/src/lib.rs:95

--- 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 ReadPipeRequest.offset in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/pipe.rs:42
  field WritePipeRequest.offset in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/pipe.rs:60
  field WritePipeRequest.length in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/pipe.rs:62
  field ReadPipeResponse.read in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/pipe.rs:51

--- failure copy_impl_added: type now implements Copy ---

Description:
A public type now implements Copy, causing non-move closures to capture it by reference instead of moving it.
        ref: https://github.com/rust-lang/rust/issues/100905
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.49.0/src/lints/copy_impl_added.ron

Failed in:
  litebox_broker_protocol::pipe::ReadPipeResponse in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/pipe.rs:49
  litebox_broker_protocol::pipe::WritePipeRequest in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/pipe.rs:56

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

Failed in:
  field data of struct ReadPipeResponse, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-wdcui_ulitebox_broker_pipe_shared_protocol/b375c5aa88bef099f1075894100a0735d13f75df/litebox_broker_protocol/src/pipe.rs:45
  field data of struct WritePipeRequest, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-wdcui_ulitebox_broker_pipe_shared_protocol/b375c5aa88bef099f1075894100a0735d13f75df/litebox_broker_protocol/src/pipe.rs:54