feat: magic in headers; proper atomic sync

[OliverNChalk]

Problem

• It's possible to try to join an SPSC queue file as an MPMC queue participant and vice versa. This will lead to the bytes being interpreted incorrectly - likely leading to panics or UB.

Solution

• Similar to rts-alloc, let's store a 64 bit magic value in the headers. We also update the code to write this value last as an atomic commit of the header.