Group synthetic events at end of EventType enum

Move Invalid, Fatal, and BracketedPasteBegin/End into the same
synthetic block as Resize, Start, Load, etc. The single boundary
(>= Invalid) now covers every non-user event, replacing the explicit
list in the keyChan activity check.

BracketedPaste markers sit in the synthetic block because the paste
content itself arrives as Rune events and updates lastActivity.

src/tui/tui.go

@@ -196,11 +196,6 @@ const (
 	CtrlAltShiftPageUp
 	CtrlAltShiftPageDown
 
-	Invalid
-	Fatal
-	BracketedPasteBegin
-	BracketedPasteEnd
-
 	Mouse
 	DoubleClick
 	LeftClick
@@ -214,7 +209,15 @@ const (
 	PreviewScrollUp
 	PreviewScrollDown
 
-	// Events
+	// Synthetic / non-user events. Everything from Invalid onward is
+	// either internally generated or a state-change notification, not
+	// direct user input. Use `>= Invalid` to gate activity tracking.
+	// BracketedPasteBegin/End sit here too: they enclose user input
+	// (which arrives as Rune events) and should not appear in FZF_KEY.
+	Invalid
+	Fatal
+	BracketedPasteBegin
+	BracketedPasteEnd
 	Resize
 	Change
 	BackwardEOF