feat(docs): add live Rhai diagram editor + PowerShell docserve

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Summary

• restore mdBook Rhai diagram sections to the supported mini-DSL
• add a live docs editor/preview for Rhai -> Mermaid with runtime diagnostics
• lazy-load Mermaid in-browser when unavailable and improve render error messaging
• add unit tests for the live editor core parser/diagnostics
• add Windows/WSL docs startup scripts (docserve-live.pwsh + docserve-live.ps1) and just wsl2-pwsh-docserve
• update AGENTS/README/visualize docs for the new workflow

Validation

• cargo test -p mdbook-rhai-mermaid
• just docgen-check
• just wsl2-pwsh-docserve
• PowerShell HTTP checks for http://127.0.0.1:3000/theory.html (200 + live assets present)