Add broker-coordinated child process startup

[Weidong Cui (wdcui)]

This PR adds broker-coordinated dynamic child process startup for userland runners by introducing broker-owned process and thread identities, inherited object references, bounded startup data, and a portable process-launcher contract. Root and child runners use the same precreated-process negotiation, authenticated association, startup deadline, supervision, and retirement path across Linux and Windows, while parent requests synchronously receive the child identity after association activation and broker shutdown waits for all launched processes to finish.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- 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/7515d7c16b81514e19b90fb58c9c48ac859bc3a0/litebox_broker_core/src/process.rs:160
  BrokerProcess::finish, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/7515d7c16b81514e19b90fb58c9c48ac859bc3a0/litebox_broker_core/src/process.rs:230

--- 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::create_process takes 1 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/7515d7c16b81514e19b90fb58c9c48ac859bc3a0/litebox_broker_core/src/lib.rs:310, but now takes 2 parameters in /home/runner/work/litebox/litebox/litebox_broker_core/src/lib.rs:334

--- 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:223

--- failure type_mismatched_generic_lifetimes: type now takes a different number of generic lifetimes ---

Description:
A type now takes a different number of generic lifetime parameters. Uses of this type that name the previous number of parameters will be broken.
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/type_mismatched_generic_lifetimes.ron
Failed in:
  Struct BrokerHostAssociation (1 -> 0 lifetime params) in /home/runner/work/litebox/litebox/litebox_broker_host/src/lib.rs:86

--- 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:StartChildProcess 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/7515d7c16b81514e19b90fb58c9c48ac859bc3a0/litebox_broker_userland/src/runtime.rs:61

--- failure struct_missing: pub struct removed or renamed ---

Description:
A publicly-visible struct cannot be imported by its prior path. A `pub use` may have been removed, or the struct 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/struct_missing.ron

Failed in:
  struct litebox_broker_userland::runner::RunnerInstance, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/7515d7c16b81514e19b90fb58c9c48ac859bc3a0/litebox_broker_userland/src/runner.rs:72

--- 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_shim_linux::LinuxShim::load_program takes 4 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/7515d7c16b81514e19b90fb58c9c48ac859bc3a0/litebox_shim_linux/src/lib.rs:263, but now takes 5 parameters in /home/runner/work/litebox/litebox/litebox_shim_linux/src/lib.rs:262