fix(update): put back the two things the PowerShell helper did

Operators are still losing the relaunch, so I went and read the code
from before 0430aab — the commit that removed the helper — instead of
theorising again. It was:

  Wait-Process -Id <pid>; Start-Sleep -Milliseconds 400;
  Start-Process -FilePath <exe> -ArgumentList '--post-update'

Two things in there that the direct launch never had:

1. Start-Process made the new OpsLog a child of PowerShell, which then
   exited — so it was detached. The direct launch makes it a child of
   the instance that is dying, in the same process group and console.
   DETACHED_PROCESS | CREATE_NEW_PROCESS_GROUP puts it back on its own,
   so nothing aimed at the old process can reach the new one.

2. It slept 400 ms AFTER the old process was gone, before starting
   anything. A process's handles are released by the kernel as it dies,
   so the mutex is free the moment the wait returns — but the things
   around it are not on that clock: the WebView2 user-data lock, the log
   file, an antivirus that woke up when the exe was replaced. This is
   not a theory about which of those it was; it is the pause being put
   back where it was.

Not restored: the PowerShell itself. Defender removed 0.27.14 from a
station as Trojan:Script/Wacatac.H!ml, and "Script/" was that helper —
an unsigned binary replacing itself and spawning a windowless script to
start another executable is, byte for byte, a dropper. Bringing it back
trades this fault for one that deletes the program.

Also: the relaunch now logs the child's pid, which the new instance's
startup.log already records on the other side. Without the pair there is
no telling "the new instance never started" from "it started and gave up
waiting", and those have different causes.

proc_detach_windows.go

@@ -0,0 +1,27 @@
+//go:build windows
+
+package main
+
+import (
+	"os/exec"
+	"syscall"
+)
+
+// detachProcess starts the child on its own, the way PowerShell's Start-Process
+// left the relaunched OpsLog.
+//
+// DETACHED_PROCESS gives it no console of ours, CREATE_NEW_PROCESS_GROUP takes
+// it out of our group so a Ctrl-Break or a job-object cleanup aimed at the old
+// instance cannot reach the new one. Deliberately NOT CREATE_NO_WINDOW and NOT
+// HideWindow: SW_HIDE in the STARTUPINFO is what made the updated OpsLog start
+// invisibly, and there is no console to suppress — OpsLog is linked for the
+// Windows GUI subsystem.
+func detachProcess(cmd *exec.Cmd) {
+	const (
+		detachedProcess       = 0x00000008
+		createNewProcessGroup = 0x00000200
+	)
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		CreationFlags: detachedProcess | createNewProcessGroup,
+	}
+}