test: preserve child exit codes on Windows PowerShell 5.1

[qtcqtc]

Closes #191.

Windows PowerShell 5.1 returns a null ExitCode for a Start-Process -PassThru child when stdout and stderr are redirected. That makes the readiness-timeout regression test fail even though the launcher exits with the expected status and diagnostic.

This change:

• starts the launcher through System.Diagnostics.ProcessStartInfo, which preserves the child exit code on Windows PowerShell 5.1 and PowerShell 7;
• reads stdout/stderr asynchronously to avoid pipe deadlocks;
• terminates the fake never-ready companion before collecting output, because it inherits the redirected handles;
• keeps the existing 15-second process bound and 10-second elapsed-time assertion.

Verified on Windows 11 / Windows PowerShell 5.1:

Windows block-monk fallback tests passed.
readiness_timeout_status=pass elapsed=4.54s

git diff --check also passes.