Add methods evidence matrix to statistics portfolio page

[grant-mccurdy]

Task\nAdd a compact methods index/table to the Statistical Methods Evidence page that maps demonstrated methods to the specific Exam 1 or Final project artifact where reviewers can inspect them.\n\n## Acceptance checks\n- Methods are grouped or labeled by source project: Exam 1 / nonlinear signal modeling and Final / clinical risk GLM.\n- Each row names the method and an inspectable artifact in the graduate-statistics-portfolio repo.\n- The page remains public-safe and does not expose private coursework prompts, source documents, private data, or protected records.\n- Local site validation passes.\n\n## Public-release risks\nLow. This should add only public-safe method names and links to already-public sanitized artifacts.