Integrate Repomix repository packing

[Adlgr87]

Motivation

• Make the repository easy for AI assistants to inspect as a single, structured bundle while keeping local secrets, build artifacts and large binaries out of any generated snapshot.

Description

• Add repomix.config.json configured to produce a parseable XML bundle (repomix-output.xml) with line numbers, Git-aware sorting and security checks.
• Add repomix-instruction.md with the recommended read order and MASSIVE-specific development protocols, and add .repomixignore to exclude secrets, caches, build artifacts and binary assets from packs.
• Add a short .gitignore entry to avoid committing generated Repomix bundles and update README.md and README_ES.md to document npx --yes repomix@latest --config repomix.config.json (and the compressed variant) for generating snapshots.

Testing

• Validated repomix.config.json with python -m json.tool repomix.config.json which succeeded.
• Verified repository formatting and pre-commit checks with git diff --check which reported no issues.
• Ran a dry Repomix run with npx --yes repomix@latest --config repomix.config.json --no-files -o /tmp/massive-repomix-check.xml which completed successfully.
• Generated a full Repomix bundle with npx --yes repomix@latest --config repomix.config.json -o /tmp/massive-repomix-full.xml --quiet and confirmed output size with wc -c, which completed successfully.

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Codex Task

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Summary by cubic

Integrates Repomix to generate a single, safe XML snapshot of the repo and adds an optional Rust core with Python wrappers for two hotspots. Public APIs stay the same; speed-ups apply when the massive_rust_core extension is present.

• New Features

  • Repo packing with repomix: added repomix.config.json, .repomixignore, repomix-instruction.md, README docs, and .gitignore to exclude generated bundles.
  • Optional Rust acceleration: new crate massive-rust-core using pyo3, numpy, ndarray, exposed via massive_core.rust_core with NumPy fallbacks.
  • Wired in wrappers: active_mask_step in MassiveEngine.step and langevin_opinion_update_inplace in IntegratedSimulator.update_agents_with_langevin; behavior unchanged.
  • Tests and metadata: added tests/test_rust_core_wrapper.py, pyproject.toml, Cargo.*, and docs/rust_core_plan_ES.md.

• Migration

  • No breaking changes; works without Rust.
  • To enable Rust: pip install maturin then maturin develop --release.
  • To create a repo snapshot: npx --yes repomix@latest --config repomix.config.json (add --compress for a smaller file).

^{Written for commit 5557040. Summary will update on new commits.}