feat(shutdown): graceful drain + configurable countdown

Add ShutdownAndDrain to the channel server (issue #179 non-breaking
subset): on SIGTERM/SIGINT, force-close all active sessions so that
logoutPlayer runs for each one (saves character data, cleans up stages
and semaphores), then poll until the session map empties or a 30-second
context deadline passes.  Existing Shutdown() is unchanged.

Add ShutdownCountdownSeconds int config field (default 10) alongside
DisableSoftCrash so operators can tune the broadcast countdown without
patching code.  A zero value falls back to 10 for safety.

Fix pre-existing test failures: MsgMhfAddRewardSongCount has a complete
Parse() implementation so it no longer belongs in the "NOT IMPLEMENTED"
parse test list; its handler test is updated to pass a real packet and
assert an ACK response instead of calling with nil.

server/channelserver/handlers_reward_test.go

@@ -87,13 +87,17 @@ func TestHandleMsgMhfAddRewardSongCount(t *testing.T) {
 	server := createMockServer()
 	session := createMockSession(1, server)
 
-	defer func() {
-		if r := recover(); r != nil {
-			t.Errorf("handleMsgMhfAddRewardSongCount panicked: %v", r)
-		}
-	}()
+	pkt := &mhfpacket.MsgMhfAddRewardSongCount{AckHandle: 42}
+	handleMsgMhfAddRewardSongCount(session, pkt)
 
-	handleMsgMhfAddRewardSongCount(session, nil)
+	select {
+	case p := <-session.sendPackets:
+		if len(p.data) == 0 {
+			t.Error("Response packet should have data")
+		}
+	default:
+		t.Error("No response packet queued")
+	}
 }
 
 func TestHandleMsgMhfAcquireMonthlyReward(t *testing.T) {
@@ -202,7 +206,6 @@ func TestEmptyHandlers_MiscFiles_Reward(t *testing.T) {
 		name string
 		fn   func()
 	}{
-		{"handleMsgMhfAddRewardSongCount", func() { handleMsgMhfAddRewardSongCount(session, nil) }},
 		{"handleMsgMhfAcceptReadReward", func() { handleMsgMhfAcceptReadReward(session, nil) }},
 	}