Document foregroundTracker unavailable-state contract

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/evidence/foreground_tracker.go

@@ -12,6 +12,8 @@ type foregroundTracker struct {
 
 // changed reports whether (available, hwnd, title) differs from the last
 // observation and records the new values. The first call always returns true.
+// Callers pass the zero values (hwnd 0, title "") when available is false, so a
+// steady "no foreground window" run does not re-emit.
 func (t *foregroundTracker) changed(available bool, hwnd uintptr, title string) bool {
 	if t.primed && available == t.available && hwnd == t.hwnd && title == t.title {
 		return false