feat(base): what prosperity needs to drop the STL

[Force67]

Everything base lacked for prosperity to replace its STL use, plus two bugs found on the way. Each change has tests; the container and threading ones also pass under ASan/UBSan and TSan.

New:

• HashMap / HashSet: chained, node-based, the std::unordered_map contract (stable references, iterator find/erase, emplace). UnorderedMap/StableMap stay for the pointer-returning style.
• SharedPointer (MakeShared, thread-safe count), RecursiveMutex, BitCast.
• SpawnDetachedThread, IsCurrentThread, Thread::handle(). Windows halves written but not compiled here.
• ConditionVariable::WaitFor/WaitUntil with a predicate.
• Map/Set lower_bound/upper_bound and operator--; Vector::insert(T&&); CopyN, NoneOf, EraseIf on node containers, Min/Max over a list, lexicographic Array/Vector operator<, Prev/Next, is_pointer_v, remove_const_t.
• ArraySize is constexpr and accepts const arrays, so it can size an Array.

Perf:

• ConditionVariable skips the wake syscall when nobody waits (waiter count next to the futex word). Prosperity's render queue notifies per submit, mostly with no one waiting.

Fixed:

• Set's iterator dropped the ancestor stack when it stepped into a right subtree: a 100-element set iterated 3 elements. Set now walks the tree like Map and has a std::set-shaped insert/find/erase.