Render readable Mermaid diagrams in the documentation

[ancongui]

The product lifecycle and AS-IS handoff guides show Mermaid source instead of diagrams. Register the custom SuperFences fence so Material renders them through its native Mermaid runtime. Keep the diagram content and change the layout to readable vertical flows, including the AS-IS fork on phones.

Browser checks enumerate every Mermaid page, inspect actual SVGs through Material's shadow roots, and cover light/dark appearance, theme changes, instant navigation, history and mobile widths. Run the fixture builder through the managed documentation environment so it uses the installed MkDocs rather than a system Python without it. The image-readiness helper visits each lazy image and checks its selected source and native dimensions before decoding it.

Validation: final make docs-check passed the strict build, discovery of 53 pages/cards, static checks, 15 Python checks and all 39 browser cases. Native diagram labels passed the 12-pixel minimum and current desktop/phone captures were inspected. Previous gallery failures and the system-Python fixture failure are retained in private qualification evidence; no browser assertions or image-loading attributes were weakened. This documentation fix does not publish the separate software candidate.