Mirrors `WeakHashSet` for map shapes. Cell-typed key and/or value
slots are stored as `Weak<T>` so entries vanish when their referent
is collected; non-cell slots are stored directly.
Similar to GC::Root<T>, make GC::RootVector<T> constructible without
explicitly passing a Heap.
This is implemented by having RootVectorBase use GC::Heap::the() for
heap-free construction.
The LibGC container and visitor tests share one heap per test binary.
A function-local static heap runs its destructor during C++ static
teardown. That makes sanitizer-only results depend on process shutdown
ordering.
Use NeverDestroyed for these process-lifetime test heaps. This matches
the intended lifetime and avoids destructor-time GC work during final
LSan checking.
