Add synchronized allocator variants

[antono3]

Summary

• add the optional antono2.memory.concurrent submodule with synchronized range and buddy allocators
• preserve the complete core allocator APIs while protecting mutations with write locks and snapshots with read locks
• add held-allocation and deterministic contention tests that join every worker thread
• add a runnable concurrent example and execute it on Linux, macOS, and Windows
• add a focused ThreadSanitizer gate, using -gc none to avoid V 0.5.2's Boehm signal-handler conflict
• prepare the additive v1.2.0 release

Validation

• official V 0.5.2 formatting and vet
• 7/7 normal test suites
• all public examples
• deterministic benchmark smoke
• 7/7 AddressSanitizer and UndefinedBehaviorSanitizer suites
• ThreadSanitizer concurrent suite with no races or leaked worker threads
• generated public API documentation