Stablize research stack and support Paper 1 scientific claims

[divyang4481]

This PR transitions the X-Theta Framework from a research prototype to a robust, reproducible research stack for Paper 1.

Key changes include:

• Scientific Corrections: Updated terminology to correctly distinguish between purity preservation and concurrence variation.
• Advanced Diagnostics: Added compute_purity and compute_tensor_spectrum to the quantum engine.
• Theory Validation: Numerically verified the analytic X-Theta correlation tensor against the evolved quantum state.
• Random CHSH Simulation: Implemented uniform Bloch sphere sampling to demonstrate the Horodecki maximum as an upper envelope over measurement settings.
• Open Data Pipeline: Ported advanced research logic (signed-area loop detection, factor encoding, streaming aggregators) into the core xtheta.data package.
• Reproducibility: Added a one-command artifact generator script (scripts/generate_paper1_artifacts.py) to reproduce all CSVs, figures, and validation reports.
• Stability: Cleaned import paths, expanded benchmark scenarios, and improved the test suite (now 33 tests).
• Maintenance: Resolved repository bloat by excluding generated artifacts and binary files via .gitignore.

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PR created automatically by Jules for task 7277917011299315192 started by @divyang4481

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