external/server: close driver.stderr.log on Stop so Windows can delete it

[mn-ram]

Closes #3736.

Symptom (from the issue)

On Windows, after stopping a WSL2 instance, limactl rm --force <inst> fails with:

failed to remove "C:\Users\…\.lima\<inst>": remove
C:\Users\…\.lima\<inst>\driver.stderr.log:
The process cannot access the file because it is being used by another process.

Root cause

server.Start in pkg/driver/external/server/server.go opens driver.stderr.log via os.OpenFile and hands the *os.File to the external-driver subprocess as cmd.Stderr:

logFile, err := os.OpenFile(logPath, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0o644)
...
cmd.Stderr = logFile

The handle is never closed in the parent (limactl). On POSIX, unlink is fine while an FD is held open; on Windows, any open HANDLE blocks DeleteFile. server.Stop sent SIGTERM but never Wait-ed the subprocess and never closed the inherited log file, so by the time the cleanup code in limactl rm --force tried to delete the instance directory, the parent process still held the file open and Windows refused to delete it.

Affects every external driver on Windows (wsl2, qemu, vz, krunkit), but the user-visible damage is worst on wsl2 because that's the Windows default.

Fix

1. Add LogFile *os.File to registry.ExternalDriver so Stop can find the handle.
2. In Stop, after sending SIGTERM, call cmd.Wait() (so the kernel reaps the child and detaches its end of the inherited handle), then explicitly Close() the parent's handle.
3. In Start, add a defer that closes logFile on every failure path between OpenFile and the success-path assignment to extDriver.LogFile; the defer is disarmed (logFile = nil) once ownership is transferred. Without this, every failed Start was leaking the FD.

+32 / 0, two files. No API change, no architecture change.

Test plan

• gofmt -l pkg/driver/external/server/server.go pkg/registry/registry.go — clean
• go vet ./pkg/driver/external/... ./pkg/registry/... — clean
• go build ./pkg/driver/external/... ./pkg/registry/... ./cmd/limactl/... — clean
• GOOS=windows GOARCH=amd64 go build ./pkg/driver/external/... ./pkg/registry/... — clean
• go test ./pkg/registry/... — pass
• CI: Windows tests (WSL2, QEMU)
• Manual on a Windows host:

  limactl start --vm-type=wsl2 --name=demo template://default-windows
  limactl stop demo
  limactl rm --force demo
  # Before: fails with "The process cannot access the file because it is being used by another process"
  # After: succeeds and the instance directory is fully removed.