Add dynamic child process startup

[Weidong Cui (wdcui)]

This PR adds broker-coordinated dynamic child process startup for userland runners: parent requests reserve child process identity and inherited broker handles, authenticated prepared-runner associations receive bounded bootstrap data, and child execution is gated on parent acknowledgement with rollback on launch failure or disconnect. It wires the transaction through Linux and Windows userland transports and runners, drains committed descendants after root completion, and adds protocol, broker-core, and end-to-end coverage.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

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

Failed in:
  BrokerProcess::parent_id, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/080368d2b1b350596297fbe098e3b7a20a43d63d/litebox_broker_core/src/process.rs:160
  BrokerProcess::finish, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/080368d2b1b350596297fbe098e3b7a20a43d63d/litebox_broker_core/src/process.rs:230

--- failure function_parameter_count_changed: pub fn parameter count changed ---

Description:
A publicly-visible function now takes a different number of parameters.
        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/function_parameter_count_changed.ron

Failed in:
  litebox_broker_host::setup_connection now takes 8 parameters instead of 5, in /home/runner/work/litebox/litebox/litebox_broker_host/src/lib.rs:227

--- failure enum_struct_variant_field_added: pub enum struct variant field added ---

Description:
An enum's exhaustive struct variant has a new field, which has to be included when constructing or matching on this variant.
        ref: https://doc.rust-lang.org/reference/attributes/type_system.html#the-non_exhaustive-attribute
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/enum_struct_variant_field_added.ron

Failed in:
  field initial_thread_id of variant BrokerHandshakeResponse::Negotiated in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/message.rs:152
  field startup of variant BrokerHandshakeResponse::Negotiated in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/message.rs:154

--- 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:ProcessStarted in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/message.rs:246
  variant BrokerOperation:StartProcess in /home/runner/work/litebox/litebox/litebox_broker_protocol/src/message.rs:69

--- failure function_missing: pub fn removed or renamed ---

Description:
A publicly-visible function cannot be imported by its prior path. A `pub use` may have been removed, or the function itself 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/v0.50.0/src/lints/function_missing.ron

Failed in:
  function litebox_broker_userland::runtime::serve_association, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/080368d2b1b350596297fbe098e3b7a20a43d63d/litebox_broker_userland/src/runtime.rs:61