Fix /setspeedmod: apply per-character, persist across stat recalculation

Previously stomped the shared GServer->MOVE_SPEED_MOD movement-timing
divisor, which character.cpp's UpdatePosition also uses for monster position
interpolation -- GM testing broke movement/attack range server-wide until
restart. Now stores a per-character SpeedModPercent that GetMoveSpeed()
applies on every recompute, so it survives future SetStats() calls instead
of being silently overwritten back to base speed. Also fixes Mspd_base (the
value actually broadcast to clients) never being set while WALKING.

Src/World Server/gmcmds.cpp

@@ -3814,10 +3814,27 @@ bool CWorldServer::pakGMChangeMonsterDmg(CPlayer* thisclient, int rate)
 	return true;
 }
 
-//change the mSpeedModif || For test only
+//change this GM's own move speed by a percentage (100 = unmodified) || For test only
 bool CWorldServer::pakGMChangeMSpeedModif(CPlayer* thisclient, int modif)
 {
-    GServer->MOVE_SPEED_MOD = modif;
+    // NOTE: this used to stomp GServer->MOVE_SPEED_MOD, a server-wide
+    // constant that character.cpp's UpdatePosition also uses for monster
+    // position interpolation -- overwriting it here broke movement/attack
+    // range for everyone until the next restart reloaded it from
+    // list_config.movespeedmod. Use a per-character percent instead, which
+    // GetMoveSpeed() applies on every recompute so it survives future stat
+    // refreshes (equip changes, buffs, etc.) instead of being a one-shot
+    // value that the next SetStats() call overwrites back to base.
+    if(modif == 0) modif = 100;
+    thisclient->Stats->SpeedModPercent = modif;
+    thisclient->SetStats( );
+
+    BEGINPACKET( pakspd, 0x782 );
+    ADDWORD( pakspd, thisclient->clientid );
+    ADDBYTE( pakspd, thisclient->Status->Stance );
+    ADDWORD( pakspd, thisclient->Stats->Mspd_base );
+    SendToVisible( &pakspd, thisclient );
+
     BEGINPACKET (pak, 0x702);
     ADDSTRING(pak, "mSpeedModif Changed.");
     ADDBYTE(pak, 0);