Mermaid diagrams: WCAG 2.2 AA contrast sweep

[ashleyshaw]

Goal

Update every Mermaid diagram in the repo so it remains readable on light backgrounds and meets WCAG 2.2 AA contrast expectations.

Acceptance criteria

• Every Mermaid diagram has accTitle and accDescr
• Mermaid style/classDef colours use the approved accessible palette
• Validation scripts cover syntax, accessibility, and contrast across the repo
• Workflow/docs guidance reflects the updated Mermaid rules

Notes

This issue tracks the repository-wide Mermaid accessibility pass and validation updates.

[linear-code]

LS-1066

[github-actions]

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