dsh-visualization v0.2.6

• Show a neutral "rendering…" status for offscreen lazy previews instead of the failure label, so unscrolled diagrams no longer read as "无法安全渲染此 Mermaid 图表" before their first render attempt.
• Reserve the failure label for post-attempt errors; real Mermaid parse failures keep their detailed message.
• Apply to both Mermaid and Vega-Lite frames.
• All existing Mermaid, table, and Vega-Lite behavior unchanged.