fix(benchmark): adapt Measure-CIBenchmark to Linux host

Measure-CIBenchmark.ps1:
- Default params now Linux-aware via $IsWindows conditional
  (TemplatePath, CloneBaseDir, VmrunPath, OutputDir)
- Add -GuestOS Auto/Windows/Linux param; Auto reads VMX guestOS field
  (ubuntu-* → Linux, else Windows)
- Replace Test-NetConnection (Windows-only) with Test-TcpPort helper
  using raw TcpClient async connect — works on PS5.1 and PS7/Linux
- Probe SSH/22 for Linux guests, WinRM/5986 for Windows guests
- Rename winrm→ready throughout ($t, JSONL field readySec, column header)
- Add guestOS field to JSONL output

_Common.psm1 / Invoke-VmrunBounded:
- Replace taskkill /F /T with platform-aware kill:
  Windows → taskkill; Linux → Process.Kill(entireProcessTree) (.NET 5+)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/_Common.psm1

@@ -182,7 +182,12 @@ function Invoke-VmrunBounded {
         # handles held by child processes (e.g. cmd.exe spawning ping.exe in tests,
         # or vmrun spawning helper subprocesses). Plain proc.Kill() only kills the
         # direct process; child processes keep the write end of stdout open.
-        & taskkill /F /T /PID $proc.Id 2>&1 | Out-Null
+        if ($null -ne $IsWindows -and $IsWindows -eq $false) {
+            # .NET 5+ Kill(entireProcessTree) — works on Linux/macOS.
+            $proc.Kill($true)
+        } else {
+            & taskkill /F /T /PID $proc.Id 2>&1 | Out-Null
+        }
     }
     $sw.Stop()