Add optional mimalloc allocator support for Release builds

[asn5d]

This enhancement proposes adding optional support for using mimalloc as the global memory allocator when building cvutil in Release mode.

Motivation

• Improve allocation performance and reduce fragmentation in long-running workloads.
• Provide more robust memory diagnostics for production builds.
• Allow developers to choose between the system allocator and mimalloc through a CMake option.
• Complements recent memory safety improvements and ASAN Debug configuration.

Proposed implementation

• Add a CMake option (e.g. USE_MIMALLOC).
• Link mimalloc and configure it as a global allocator when enabled.
• Keep feature on by default.
• Maintain full backward compatibility with existing builds.

Notes

• This enhancement is not required to fix issues Segmentation fault in bwthin #2 and Integer overflow in bwthin #3.
• It is a standalone improvement following memory safety work.
• PR will follow after local testing.

[asn5d]

The feature is implemented in PR #6.