Enable shared broker pipes over Unix sockets

[wdcui]

Summary

• create one sealed memfd per broker pipe and transfer it with SCM_RIGHTS
• map separate read and write staging regions on both control-channel peers
• allow only the Linux-userland syscalls needed to receive and validate attachments
• cover two-way mapping visibility and the runner broker integration path

Stack

1. Add broker shared-memory primitives #1049 — protocol foundation
2. Add broker-host shared pipe staging #1048 — broker-host staging
3. Add broker-local shared pipe data path #1051 — broker-local data path
4. Enable shared broker pipes over Unix sockets #1050 — Unix transport enablement

[github-actions]

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

🤖 SemverChecks 🤖 No breaking API changes detected

Note: this does not mean API is unchanged, or even that there are no breaking changes; simply, none of the detections triggered.

[wdcui]

Superseded by #1052. The generic Unix shared-memory transport now lives in #1049, while #1052 owns the shared-memory-only broker-pipe protocol and data path.