The Windows client now fails fast outside Windows before tunnel setup because
its real MVP runtime depends on TAP-Windows6 and Windows route protection. The
old non-Windows runtime branch still built a control-event loop and printed
that TAP pumping was not wired yet, which described an obsolete smoke-test path
rather than current behavior.
Replace that branch with an unreachable guard and compile the related runtime
formatting/status helpers only for Windows or unit tests. Non-Windows builds
still type-check the client, but the runtime behavior is now only the explicit
unsupported-platform error.
Test Plan:
- cargo fmt
- cargo test -p lanparty-client-win
- cargo clippy --workspace --all-targets -- -D warnings
- cargo test --workspace
Refs: MVP Windows client runtime gate
ddidderr
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