Add shared guest network routing

[Weidong Cui (wdcui)]

Summary

• add explicit broker-wide guest and gateway identity with typed guest, gateway, and external routing
• enforce guest-network, external destination, and protocol-specific gateway policy, translating authorized gateway traffic to host loopback
• route private guest TCP/UDP and runner traffic while retaining one native UDP endpoint for all destinations outside the sandbox

Custom address configuration and virtual TCP remain separate follow-up work.

Testing

• cargo fmt
• targeted build and Clippy for changed packages
• targeted Nextest and runner integration suites (333 tests)
• cargo test -p litebox_broker_userland --test userland_broker

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

Click for details

--- failure constructible_struct_adds_field: struct exhaustively constructible through public API adds field ---

Description:
A pub struct that could be exhaustively constructed with a literal using only public API has a new pub field, breaking existing exhaustive literals.
        ref: https://doc.rust-lang.org/reference/expressions/struct-expr.html
       impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.50.0/src/lints/constructible_struct_adds_field.ron

Failed in:
  field AcceptedPlatformSocket.remote in /home/runner/work/litebox/litebox/litebox_broker_core/src/socket.rs:231

--- 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:
  GuestSocketBinding::is_valid, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/d1f71bdda0758d08dc633482c44326fe6e8c769d/litebox_broker_core/src/socket.rs:257

--- 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 2 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/d1f71bdda0758d08dc633482c44326fe6e8c769d/litebox_broker_core/src/lib.rs:127, but now takes 3 parameters in /home/runner/work/litebox/litebox/litebox_broker_core/src/lib.rs:132
  litebox_broker_core::BrokerCore::new_with_limits takes 3 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/d1f71bdda0758d08dc633482c44326fe6e8c769d/litebox_broker_core/src/lib.rs:132, but now takes 4 parameters in /home/runner/work/litebox/litebox/litebox_broker_core/src/lib.rs:146
  litebox_broker_core::socket::GuestSocketBinding::covers takes 1 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/d1f71bdda0758d08dc633482c44326fe6e8c769d/litebox_broker_core/src/socket.rs:269, but now takes 2 parameters in /home/runner/work/litebox/litebox/litebox_broker_core/src/socket/network.rs:193

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

Failed in:
  field remote_address of struct AcceptedPlatformSocket, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/d1f71bdda0758d08dc633482c44326fe6e8c769d/litebox_broker_core/src/socket.rs:309

--- 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_platform_linux_userland::LinuxSocketProvider::new takes 2 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/d1f71bdda0758d08dc633482c44326fe6e8c769d/litebox_broker_platform_linux_userland/src/socket.rs:177, but now takes 3 parameters in /home/runner/work/litebox/litebox/litebox_broker_platform_linux_userland/src/socket.rs:183