Introduce VirtualStack allocator, refactor memory management to use virtual memory stacks, and update documentation.
Added VirtualStack as a new stack allocator using virtual memory, replaced Stack with VirtualStack in allocation manager and related APIs, and updated TempJobAllocator to use VirtualArena. Introduced AllocationManagerInitOpts for allocator configuration. Replaced ENABLE_COLLECTION_CHECKS with ENABLE_SAFETY_CHECKS for safety checks. Removed Result.cs and updated project files and examples. Added comprehensive README files for all major packages and improved root documentation.
BREAKING CHANGE: Stack allocator replaced by VirtualStack; TempJobAllocator and AllocationManager initialization signatures changed; Result types removed.
Added new AllocationHandle property in Stack.Scope.
Changed the ref AllocationHandle constructor parameter to AllocationHandle on of all UnsafeCollection types
Removed Allocator.Stack. Use Stack.Scope.AllocationHandle to allocate on stack instead.
