Add optional PyO3 Rust core acceleration

[Adlgr87]

Motivation

• Move hot numeric kernels out of Python into an optional Rust core to reduce CPU overhead while preserving the existing Python API.
• Target kernels that operate on dense NumPy arrays with deterministic clipping behavior (Langevin opinion update, active-mask computation, potential gradient) to minimize compatibility risk.
• Provide a safe, opt-in Python Wrapper / Rust Core design that keeps RNG in Python and falls back to NumPy when the Rust extension is not available.

Description

• Add a PyO3/Maturin Rust crate (rust_core/src/lib.rs, Cargo.toml) exposing multi_potential_gradient_rs, langevin_opinion_update_inplace, and active_mask_step_rs as deterministic numeric kernels.
• Add a stable Python wrapper module massive_core/rust_core.py that imports the compiled massive_rust_core when present and otherwise provides NumPy fallbacks for the same functions.
• Wire the wrapper into the runtime: IntegratedSimulator.update_agents_with_langevin now delegates opinion updates to langevin_opinion_update_inplace, and ActiveSet.step delegates to active_mask_step, preserving previous public APIs and semantics.
• Add packaging/build files and docs (pyproject.toml, tests/test_rust_core_wrapper.py, docs/rust_core_plan_ES.md) documenting selection criteria, migration plan, and equivalence tests.

Testing

• cargo test ran and passed the Rust unit test(s) for the crate (Rust tests: passed).
• cargo fmt / cargo fmt --check were run and the code was formatted (format check passed after formatting).
• maturin build --release produced a wheel and the wheel was installed successfully, and importing massive_rust_core succeeded (build + import smoke test: passed).
• pytest tests/test_rust_core_wrapper.py and the full Python test suite pytest tests/ were executed and all tests passed (300 passed, 1 warning).

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Codex Task

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Summary by cubic

Add an optional Rust core (massive_rust_core) to accelerate hot numeric kernels while preserving the Python API and exact results. Falls back to numpy when the extension isn’t present.

• New Features

  • Added Rust crate massive_rust_core (via pyo3/maturin) exposing multi_potential_gradient_rs, langevin_opinion_update_inplace, and active_mask_step_rs.
  • Added Python wrapper massive_core.rust_core that uses Rust if available, or numpy fallbacks otherwise (same signatures and clipping behavior).
  • Wired into runtime: IntegratedSimulator.update_agents_with_langevin and ActiveSet.step now delegate to the wrapper without changing public APIs.
  • Included docs and tests for equivalence; build config in pyproject.toml.

• Migration

  • No code changes required; NumPy is used if massive_rust_core isn’t installed.
  • To enable acceleration: pip install maturin and run maturin develop --release (or maturin build --release) to install massive_rust_core.

^{Written for commit 4cb7c72. Summary will update on new commits.}