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.

config/config.go

@@ -67,7 +67,8 @@ type Config struct {
 	Host                   string `mapstructure:"Host"`
 	BinPath                string `mapstructure:"BinPath"`
 	Language               string
-	DisableSoftCrash       bool     // Disables the 'Press Return to exit' dialog allowing scripts to reboot the server automatically
+	DisableSoftCrash            bool // Disables the 'Press Return to exit' dialog allowing scripts to reboot the server automatically
+	ShutdownCountdownSeconds    int  // Seconds to count down before shutting down (default 10; ignored when DisableSoftCrash is true)
 	HideLoginNotice        bool     // Hide the Erupe notice on login
 	LoginNotices           []string // MHFML string of the login notices displayed
 	PatchServerManifest    string   // Manifest patch server override
@@ -353,6 +354,7 @@ func registerDefaults() {
 	viper.SetDefault("CommandPrefix", "!")
 	viper.SetDefault("AutoCreateAccount", true)
 	viper.SetDefault("LoopDelay", 50)
+	viper.SetDefault("ShutdownCountdownSeconds", 10)
 	viper.SetDefault("DefaultCourses", []uint16{1, 23, 24})
 	viper.SetDefault("EarthMonsters", []int32{0, 0, 0, 0})