Since you insist on reducing the Constitution to the level of a sports contract, ...
A legal contract is a legal contract.
If you wish, I can make an analogy regarding a marriage contract, a corporate partnership contract, a real estate contract, contracted services or goods contract, another Constitution, etc. etc, .....
I'll point out that if there is no "out clause" in the contract then one doesn't exist.
There was an old joke about Germany that said that the difference between America and Germany was that, in Germany, anything that was not specifically allowed was strictly forbidden while in America, any thing that was not specifically forbidden was allowed.
The purpose of the Tenth Amendment is to enshrine that in the Constitution.
Was secession mentioned in the Constitution?
No.
In 1860, had there ever been a Federal law passed that forbid secession?
No.
What did the Constitution say about the Powers not specifically prohibited by the Constitution or prohibited by Federal law?
You don't assume that since there is no "out clause" specified then you can leave in any manner and for any reason that you want.
No. I assume that the men of 1861 believed that the Constitution said what it meant and meant what it said and the Tenth Amendment said:
You will notice that the Tenth Amendment did not come with an asterisk referring you to the fine print you wish to trump the Constitution with.
Or take team equipment with you or walk away from money owed the management.
Very true. That was something that was not guaranteed by the Tenth Amendment and the matter of property was handled atrociously by both side. The matter was certainly not worth the lives of 600,000 Americans.
For a way to do it without killing hundreds of thousands, refer to the 1997 Partition Treaty of the Soviet Fleet between Ukraine and Russia.