Route stdout and stderr through the broker

[Weidong Cui (wdcui)]

This PR routes /dev/stdout and /dev/stderr writes through a new shared-memory broker stdio operation, adds a userland broker provider for inherited output streams, requires an active broker for non-empty writes while preserving empty and bounded partial-write behavior, and updates runner tests, UnixBench integration, CI, and LVBS documentation for brokered output.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

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--- failure method_parameter_count_changed: pub method parameter count changed ---

Description:
A publicly-visible method now takes a different number of parameters, not counting the receiver (self) parameter.
        ref: https://doc.rust-lang.org/cargo/reference/semver.html#fn-change-arity
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/method_parameter_count_changed.ron

Failed in:
  litebox_broker_core::BrokerCore::new takes 3 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/3dcf9d61088d0e4fe296ebc054568bf620f102d5/litebox_broker_core/src/lib.rs:130, but now takes 4 parameters in /home/runner/work/litebox/litebox/litebox_broker_core/src/lib.rs:133
  litebox_broker_core::BrokerCore::new_with_limits takes 4 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/3dcf9d61088d0e4fe296ebc054568bf620f102d5/litebox_broker_core/src/lib.rs:144, but now takes 5 parameters in /home/runner/work/litebox/litebox/litebox_broker_core/src/lib.rs:149

--- 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/v0.50.0/src/lints/enum_variant_added.ron

Failed in:
  variant BrokerResult:Stdio in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/message.rs:155
  variant BrokerOperation:Stdio in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/message.rs:50