[tamird]

A netstack listen or forwarding port of 65537 wraps to port 1. A bad
native forwarding port also makes the proxy retry a connection that
cannot succeed.

Parse the listen port at its 16-bit width and the netstack destination
as a numeric address and port before opening network devices. Check the
native forwarding port at startup so permanent errors cannot enter the
retry loop. Leave host resolution to each connection attempt.

[manninglucas]

LGTM. The change is small, correct, and merges cleanly with master. It keeps behaviour for valid inputs: an IPv4 target still reaches netstack as a v4-mapped address and is unmapped in AddrNetProtoLocked, as before. Optional notes:

1. Commit message: the change also fixes the swapped -client/-server help strings (client = netstack dial, server = netstack listen) and moves to pointer receivers. Please add a sentence about the help-string fix so it doesn't look accidental.
2. Native forward port (optional): with -forward host:65537 and no -client, net.Dial fails with "invalid port" on every attempt, so the pre-bind loop still retries forever. net.SplitHostPort + net.LookupPort("tcp", port) up front would reject it without giving up per-attempt DNS. This would also make the description's second sentence true for both paths.
3. Please confirm a normal tcp_benchmark.sh --client --server run still works, since this binary has no automated test coverage.

PR description / commit message:
Please also mention that the -client/-server help strings were swapped and are fixed here (client = netstack dial, server = netstack listen). The current description doesn't cover it.

Security: No security impact: the change only touches the out-of-sandbox tcp_proxy benchmark binary and makes its flag parsing stricter (16-bit port parse, netip.ParseAddrPort for the netstack target). It adds no host interactions, no seccomp or Sentry changes, and no paths reachable from sandboxed code or remote peers.

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[🤖] This review is part of an experimental auto-review process to help speed up gVisor review velocity. It is a work in progress. Assisted-by: Claude