Share broker TCP namespace

[Weidong Cui (wdcui)]

This PR gives all broker sessions one shared guest TCP namespace by moving guest TCP port reservation and ephemeral allocation into broker core and routing guest-to-guest connections through private Linux endpoints with guest-visible accept translation. It adds platform-owned TCP lifecycle tracking, bounded pending and stale-connection state, retained connector accounting, backlog-drain safeguards, and blocking-accept readiness coverage, and updates the Linux runner TCP server integration to use the shared namespace.

[github-actions]

🤖 SemverChecks 🤖 ⚠️ Potential breaking API changes detected ⚠️

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--- 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 local_address of struct AcceptedPlatformSocket, previously in file /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/198d7bd7c8de65efe55d8c397492a3d2e7d4623f/litebox_broker_core/src/socket.rs:31

--- 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 1 parameters in /home/runner/work/litebox/litebox/target/semver-checks/git-ulitebox/198d7bd7c8de65efe55d8c397492a3d2e7d4623f/litebox_broker_platform_linux_userland/src/socket.rs:157, but now takes 2 parameters in /home/runner/work/litebox/litebox/litebox_broker_platform_linux_userland/src/socket.rs:159