Expose synchronized allocators from root module

[antono3]

Summary

• expose SynchronizedRangeAllocator and SynchronizedBuddyAllocator from the root antono2.memory import
• keep the lightweight RangeAllocator and BuddyAllocator constructors unchanged as the default path
• preserve the v1.2 antono2.memory.concurrent API as source-compatible aliases
• document and demonstrate the recommended root-module API

Validation

• released V 0.5.2 (7647ce1): format verification and vet
• released V 0.5.2: 8/8 test files, including legacy concurrent aliases
• released V 0.5.2: all public API examples
• local V 0.5.2: deterministic benchmark smoke test
• local V 0.5.2 + Clang: AddressSanitizer and UndefinedBehaviorSanitizer, 8/8 test files
• local V 0.5.2 + Clang: ThreadSanitizer contention suite