refactor: remove Mermaid custom action slots

[cc-hearts]

🤔 This is a ...

• 🆕 New feature
• 🐞 Bug fix
• 📝 Site / documentation improvement
• 📽️ Demo improvement
• 💄 Component style improvement
• 🤖 TypeScript definition improvement
• 📦 Bundle size optimization
• ⚡️ Performance optimization
• ⭐️ Feature enhancement
• 🌐 Internationalization
• 🛠 Refactoring
• 🎨 Code style optimization
• ✅ Test Case
• 🔀 Branch merge
• ⏩ Workflow
• ⌨️ Accessibility improvement
• ❓ Other (about what?)

🔗 Related Issues

• Follow-up to feat(mermaid): add SlotsType declaration for header and custom action slots #107.

💡 Background and Solution

Mermaid custom action slots made the header customization semantics confusing because the header slot replaces the whole header area. This removes customActionIconRender and customActionRender from the Mermaid slot API while keeping typed slots for the supported header slot.

The Mermaid docs demos have also been aligned with /Users/carl/Desktop/repo/oss/x/packages/x/components/mermaid/demo:

• Basic
• Custom Header
• Header Actions
• With XMarkdown
• Semantic DOM

Where Vue slots are available, the demos prefer slots, such as the Mermaid header slot and Bubble content slot. Custom action icons remain item data because Mermaid does not expose the underlying Actions iconRender slot.

Validation:

• vp test packages/x/components/mermaid/tests/index.test.tsx
• vp check --fix via the staged-files commit hook
• vp check (format passes; existing unrelated lint/type issues remain)

📝 Change Log

Language	Changelog
🇺🇸 English	Remove Mermaid custom action slots and align Mermaid demos with upstream examples.
🇨🇳 Chinese	移除 Mermaid 自定义操作项 slots，并对齐 Mermaid 示例与上游示例。