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.

network/mhfpacket/msg_parse_small_test.go

@@ -18,7 +18,6 @@ func TestParseSmallNotImplemented(t *testing.T) {
 	}{
 		// MHF packets - NOT IMPLEMENTED
 		{"MsgMhfAcceptReadReward", &MsgMhfAcceptReadReward{}},
-		{"MsgMhfAddRewardSongCount", &MsgMhfAddRewardSongCount{}},
 		{"MsgMhfDebugPostValue", &MsgMhfDebugPostValue{}},
 		{"MsgMhfEnterTournamentQuest", &MsgMhfEnterTournamentQuest{}},
 		{"MsgMhfGetCaAchievementHist", &MsgMhfGetCaAchievementHist{}},