Hardening: Introduce overflow-checked custom allocation primitives

[uwezkhan]

This PR introduces a safer allocation pattern across the library to prevent heap buffer overflows caused by integer overflows in nmemb * size calculations.

Infrastructure: Added ZSTD_customCalloc2 to allocations.h with robust SIZE_MAX overflow checks.
Remediation: Systematically refactored critical components (Thread Pool, MT-Compression, and Decompression DDict Hashsets) to use these safe primitives.
Critical Fix: Specifically addresses a potential 32-bit heap overflow in ZSTD_DDictHashSet_expand.
Verification: Added a unit test to validate the overflow protection logic.

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