chore: extract embedded benchmark into standalone mesa_benchmark repository

[Yasou13]

Summary

Extracts the embedded mesa-benchmark/ suite into an independent standalone repository (mesa_benchmark).

Key Changes

• Clean Architecture Boundaries: Core mesa-memory has zero dependency on mesa_benchmark. Dependency flows strictly one-way: mesa_benchmark -> installed mesa-memory.
• Packaging Integrity: Core package manifests (pyproject.toml, wheel builds) strictly exclude benchmark datasets, artifacts, and CLI entry points.
• Deterministic Equivalence: Independent validation confirmed 1.0 exact parity with previous benchmark execution and metrics.
• Full Test Suite & Quality: 1,873 core MESA unit, integration, and regression tests passing cleanly with zero regressions.
• Docs & Showcases: Updated documentation and interactive mock showcase terminals to reference standalone mesa-benchmark CLI.

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