Stop polluting FZF_KEY with synthetic events

lastKey was overwritten every time a synthetic event (every(N), Focus,
Result, ...) was processed, so FZF_KEY would briefly show "every(1)"
or empty string instead of the user's actual last keystroke.

Gate the assignment on `< Invalid` so only user-input events update
lastKey, and drop the now-unused every() formatting in KeyName.

src/options_test.go

@@ -330,10 +330,6 @@ func TestParseEveryEvent(t *testing.T) {
 		}
 	}
 
-	// KeyName round-trips with the original duration
-	if got := (tui.Event{Type: tui.Every, Char: 2000}).KeyName(); got != "every(2)" {
-		t.Errorf("KeyName: %q != every(2)", got)
-	}
 }
 
 func TestColorSpec(t *testing.T) {

src/terminal.go

@@ -6478,7 +6478,9 @@ func (t *Terminal) Loop() error {
 		previousInput := t.input
 		previousCx := t.cx
 		previousVersion := t.version
-		t.lastKey = event.KeyName()
+		if event.Type < tui.Invalid {
+			t.lastKey = event.KeyName()
+		}
 		updatePreviewWindow := func(forcePreview bool) {
 			t.resizeWindows(forcePreview, false)
 			req(reqPrompt, reqList, reqInfo, reqHeader, reqFooter)

src/tui/tui.go

@@ -257,10 +257,6 @@ func (e Event) KeyName() string {
 		return me.Name()
 	}
 
-	if e.Type == Every {
-		return "every(" + strconv.FormatFloat(float64(e.Char)/1000, 'f', -1, 64) + ")"
-	}
-
 	if e.Type >= Invalid {
 		return ""
 	}