Refresh EX-IOExpander TCP client compatibility and lifecycle coverage

[BanjoR]

Summary Refresh the TCP EX-IOExpander client work from preliminary PR #502 in the context of issue #372. ## Scope and related work - Reconcile poll-response framing with the separate EX-IOExpander TCP server PR #65, accepting tagged payloads while retaining raw-payload compatibility. - Retry partial Client::write results so frames cannot be truncated. - Add a dependency-free PowerShell loopback harness for fragmented frames, tagged/raw polls, error responses, reconnect after disconnect, and two independent client sessions. - Personal integration code and unrelated protocol behavior are explicitly out of scope. ## Validation evidence - pwsh -NoProfile -File test/tcp_client_protocol.tests.ps1 passed all 5 cases. - git diff --check and the scoped source/path audit passed. - PASS - Exact default python -m platformio run in a clean task checkout with an isolated PlatformIO core completed for all five configured environments: mega2560, ESP32, Nucleo-F411RE, Nucleo-F446RE, and Nucleo-F429ZI. - No superseded BanjoR PR was identified; PR #502 is the linked preliminary contribution being refreshed.

• N/A - Fork-hosted CI: no Actions run exists for exact head 87d8c817fc17b8424f8558de54030576e243adf4; the available CommandStation fork CI is push-triggered and did not record a run for this head.
• N/A - Upstream PR workflows for this head are Docs run 31942856855 and Label sponsors run 31942857011, both action_required with no firmware jobs; neither is build validation. ## Hardware validation Hardware validation: Not run—no hardware available. Maintainer bench criteria: connect a CommandStation and EX-IOExpander TCP server, exercise fragmented and complete tagged/raw poll frames, malformed/error responses, disconnect/reconnect, partial writes, and two independent client sessions; verify no truncation or cross-session state leakage. This PR is ready for maintainer review; network/hardware bench validation remains outstanding; local five-environment compilation is complete.

Current exact-head CI status

N/A — No BanjoR fork Actions run exists for exact head 87d8c817fc17b8424f8558de54030576e243adf4; no hosted code-test result is claimed. Local validation above is the available evidence.