C4 图: 统一渲染器迁移和 c4-beta - PR 系列跟踪器

This issue tracks the C4 diagram modernization effort, following the PR-split approach requested by @aloisklink in https://GitHub.com/mermaid-js/mermaid/pull/7673#pullrequestreview and the TS-conversion handoff by @sidharthv96 in https://GitHub.com/mermaid-js/mermaid/pull/7829#issuecomment-4686299579. The goal: replace the legacy row-grid C4 renderer with the unified rendering pipeline (real layout via dagre/ELK, themes, tags, icons, links, legend) - plus a new structurizr-inspired `c4-beta` syntax (RFC: https://GitHub.com/orgs/mermaid-js/discussions/7844).