The power to create a state and, once admitted, approve changes in its status is a power reserved to the United States in Article IV, Section III and Article I, Section 10. By implication this would include leaving the Union altogether. The power to act in a unilateral manner where the interests of the other states may be impacted is a power denied the states in Article I, Section 10. By implication this would include impact that a state leaving the Union would have on the other states.
You admit the Constitution does not explicitly forbid secession, therefore it is reserved to the States and the people.
Powers do not have to be explicit to exist. The Supreme Court recognized the existence of implied powers almost 200 years ago.
Thay overlook the Preamble to the Bill of Rights:
The conventions of a number of the States having at the time of their adopting the Constitution, expressed a desire, in order to prevent misconstruction or abuse of its powers, that further declaratory and restrictive clauses should be added: And as extending the ground of public confidence in the Government, will best insure the beneficent ends of its institution.