docs: align production roadmap with current engine state

[JeanPhilippeKernel]

Summary

• align the roadmap, architecture references, and active plans with the current engine and issue-tracker state
• move completed implementation records into ZEngine/docs/completed and retain concrete production acceptance gates for remaining work
• document the Linux root-arena overcommit limitation and the required POSIX reserve/commit redesign
• repair Mermaid labels that used parser-hostile C++ template syntax

Validation

• git diff --check origin/develop...HEAD
• local relative Markdown-link audit
• Mermaid-fence audit for HTML entities and template labels
• pre-push clang-format hook (all source trees)