gh-118: Feature/dark emulator

[ryusei172525]

📌 Related Issue

Closes #118

✨ What's been added or changed?

• tutorials/dark_emulator/ (new): Intro and observables-usage notebooks; fitting notebooks for GC (w_p), GGL (ΔΣ), and 3x2pt (mock data + Nautilus). README with notebook list, production commands, and optional video link.
• scripts/sampling/darkemu_3x2pt_full_sampling.py (new): Production 3x2pt run (mock data, scale cuts, Nautilus, Omega_m/sigma8/S8 in npz). Safe to import from Jupyter.
• scripts/plotting/corner_plot.py (new): Corner plots from Nautilus chain npz (e.g. Omega_m, sigma8, S8).
• playground/README.md: Dark Emulator in Features and Usage (notebooks + production script).

🧪 How was it tested?

• Ran fitting notebooks and imported the sampling script from a notebook; checked README commands.

📸 Screenshots (if applicable)

🧠 Anything to keep in mind?

• Production 3x2pt: use the sampling script (e.g. n_live=500) then corner_plot.py on the saved npz; the 3x2pt notebook is a short demo only.
• There is a short video covering the various notebooks.

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• I've named the title of this pull request starting by 'gh-#: TITLE'
• I've linked the related issue
• I've tested the code or notebooks manually
• I've added comments/docstrings where needed (i.e: README)
• I've updated relevant docs if necessary

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