Carreau walkthrough: Cox–Merz and Delaware–Rutgers validation (closes #25)

[marcocaggioni]

Closes #25.

New case study docs/walkthrough-carreau.md on a_remake_dhr2.json (high-MW linear polymer in aqueous solution, 25 °C):

• Carreau fit of the flow curve: η₀ = 1.99 Pa·s, λ = 0.199 s, n = 0.414 (all ≤6% error, RedChi² = 3.85e-3, cond 9.5), with fit + residuals plot
• Cox–Merz validation: |η*(ω)| from the frequency sweep overlays η(γ̇) to within 2.3% over three decades (Cox & Merz 1958)
• Delaware–Rutgers validation: η′(γ₀ω) from the amplitude sweep overlays η(γ̇) to within 9% (Doraiswamy et al. 1991); amplitude sweep also confirms the frequency sweep's 0.5% strain is inside the LVE
• Mermaid workflow diagram (renders on HTML via Render mermaid diagrams on Read the Docs #24's sphinxcontrib-mermaid; SVG fallback for EPUB)
• All three references verified with clickable DOIs

Verified locally: sphinx-build -W clean, diagram renders on HTML, SVG fallback in EPUB, all figures referenced.