Tighten docstrings: drop em-dashes and AI-ish filler

[bwengals]

A docstring and comment cleanup across ptgp/ to make the prose read less like an AI wrote it. No code, signatures, or behavior change.

What changed

• Removed every em-dash (the "—" character) from docstrings and comments. The repo now has zero. Replacements are periods, commas, semicolons, or colons as reads best.
• Dropped marketing/filler phrasing ("Batteries-included", "suitable for use with", a stray "simply", a "Useful for").
• Kept all math, formulas, tensor shapes, parameter/return descriptions, and the non-obvious caveats and warnings (jitter/PSD rationale, NaN-gradient clamp, FITC "not a lower bound", condition-number health notes, VFF out-of-domain caveats, scipy/shared-variable interop notes, and so on).

18 files touched, a near 1:1 line swap (61/61). Pure docstring/comment edits.

Test plan

• pytest tests/ 265 passed
• pre-commit run --all-files clean (ruff 0.5.5 + ruff-format)
• scripts/run_mypy.py 37/37 pass
• grep -rn "—" ptgp/ returns nothing

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📚 Documentation preview 📚: https://ptgp--41.org.readthedocs.build/en/41/