docs(napkin-math): add methodology doc with assessment-slideshow content

[neoneye]

Summary

Adds experiments/napkin_math/docs/methology.md describing how the napkin-math Monte Carlo works end-to-end: the 8 pipeline stages, single-run sampling discipline, where bounds come from (with the data vs assumption source label), where thresholds come from (threshold_basis enum), and the worst-gate framing.

Includes a slideshow section with two slides intended for insertion before the plan roster in the assessment slideshow — directly addressing reviewer feedback that a skeptical reader will ask "where did the distributions come from?" and "who decided the thresholds?".

The doc is paradigm-agnostic (no LLM/AI references) so it stays accurate as the implementation evolves. Versioning is not hardcoded so the doc can carry forward across future slideshow generations.

Test plan

• Markdown renders cleanly on GitHub.
• No internal links broken (none introduced).

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