net/game: initialise entity swim/walk state from spawn packet and SPLINE_MOVE opcodes

UpdateBlock now stores moveFlags from the LIVING movement block so the
cold-join problem is fixed: entities already swimming or walking when the
client joins get their animation state correctly initialised from the
SMSG_UPDATE_OBJECT CREATE_OBJECT packet rather than waiting for the next
MSG_MOVE_* heartbeat.

Additionally, SMSG_SPLINE_MOVE_START_SWIM, SMSG_SPLINE_MOVE_STOP_SWIM,
SMSG_SPLINE_MOVE_SET_WALK_MODE, SMSG_SPLINE_MOVE_SET_RUN_MODE, and
SMSG_SPLINE_MOVE_SET_FLYING now fire unitMoveFlagsCallback_ with
synthesised flags so explicit server-driven mode transitions update
animation state immediately without waiting for a heartbeat.

include/game/world_packets.hpp

@@ -481,6 +481,10 @@ struct UpdateBlock {
     // Update flags from movement block (for detecting transports, etc.)
     uint16_t updateFlags = 0;
 
+    // Raw movement flags from LIVING block (SWIMMING=0x200000, WALKING=0x100, CAN_FLY=0x800000, FLYING=0x1000000)
+    // Used to initialise swim/walk/fly state on entity spawn (cold-join).
+    uint32_t moveFlags = 0;
+
     // Transport data from LIVING movement block (MOVEMENTFLAG_ONTRANSPORT)
     bool onTransport = false;
     uint64_t transportGuid = 0;