Stop orphaning wine subprocesses when vintner is killed by PID

Every wrapped tool invocation (cl/link/msbuild/etc via wine, plus the
native cmd/findstr shims) now starts its child in its own process
group and forwards SIGINT/SIGTERM to that group, escalating to
SIGKILL after a 5s grace period if it doesn't exit.

Previously, interactive Ctrl-C happened to work by accident (the
child inherited the terminal's foreground process group and got the
signal directly), but anything that signals vintner by PID alone - a
CI job's timeout, a supervisor's `kill <pid>` - never reached the
wine/wineserver tree underneath it, which got reparented to init and
kept running: wasted CPU, held file locks, stray FIFOs/temp files.

Verified two ways: a unit test (signals_test.go) that starts a
detached `sleep 30`, signals the test process itself, and checks the
child actually dies; and a real end-to-end run - killed an in-flight
`msbuild` driver build by PID mid-compile and confirmed no orphaned
msbuild/cl/link/vintner process was left behind (wineserver and its
persistent service processes are expected to survive, by design - see
pipeDrainGrace's doc comment).

internal/wrapper/signals.go

@@ -0,0 +1,68 @@
+package wrapper
+
+import (
+	"os"
+	"os/exec"
+	"os/signal"
+	"syscall"
+	"time"
+)
+
+// killGrace bounds how long a forwarded SIGINT/SIGTERM gets to make a
+// subprocess tree exit on its own before escalating to SIGKILL - long
+// enough for wineserver to tear down a Windows process tree cleanly, short
+// enough that an unresponsive one doesn't hang vintner's own shutdown.
+const killGrace = 5 * time.Second
+
+// setNewProcessGroup puts cmd's eventual child in its own process group
+// (pgid = its own pid) instead of inheriting vintner's. Without this, a
+// caller that signals vintner by PID alone (a CI runner enforcing a
+// timeout, a supervisor's `kill <pid>`) never reaches the wine/wineserver
+// tree underneath it, which is then reparented to init and keeps running -
+// wasting CPU, holding file locks, leaving stray FIFOs/temp files behind.
+// (Interactive Ctrl-C already reaches every process in the terminal's
+// foreground group regardless of this, but forwardSignals below handles
+// that case too now that the child has moved to its own group.)
+func setNewProcessGroup(cmd *exec.Cmd) {
+	cmd.SysProcAttr = &syscall.SysProcAttr{Setpgid: true}
+}
+
+// forwardSignals relays SIGINT/SIGTERM received by vintner itself to
+// proc's entire process group (proc must have been started via a cmd that
+// called setNewProcessGroup, making proc.Pid also the group id), escalating
+// to SIGKILL after killGrace if the group hasn't exited by then. Callers
+// must call the returned stop func once the process has actually exited
+// (e.g. right after cmd.Wait() returns), both to stop listening for
+// signals and to cancel a pending escalation.
+func forwardSignals(proc *os.Process) (stop func()) {
+	sigCh := make(chan os.Signal, 1)
+	signal.Notify(sigCh, syscall.SIGINT, syscall.SIGTERM)
+	done := make(chan struct{})
+
+	go func() {
+		pgid := -proc.Pid
+		for {
+			select {
+			case sig := <-sigCh:
+				s, ok := sig.(syscall.Signal)
+				if !ok {
+					continue
+				}
+				_ = syscall.Kill(pgid, s)
+				select {
+				case <-time.After(killGrace):
+					_ = syscall.Kill(pgid, syscall.SIGKILL)
+				case <-done:
+					return
+				}
+			case <-done:
+				return
+			}
+		}
+	}()
+
+	return func() {
+		signal.Stop(sigCh)
+		close(done)
+	}
+}