[WIP] Add support for virtual host functionality

[Copilot]

• Understand all unresolved review comments
• Fix: Remove fmt.Printf and downgrade hot-path logs to Debugf in static.go
• Fix: Path traversal in handleWebHosting - use path.Join+prefix check to prevent traversal
• Fix: Path traversal in applyVhostPathMappingWithPrefix in fsread.go - same approach
• Fix: IPv6 port stripping in stripHostPort in static.go (use net.SplitHostPort)
• Fix: IPv6 port stripping in stripHostPortForVhost in fsread.go (use net.SplitHostPort)
• Fix: IPv6 port stripping in stripDownHostPort in down.go (use net.SplitHostPort)
• Fix: gofmt formatting in static.go else branch
• Build verified, CodeQL clean (0 alerts)

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[github-actions]

⚠️ PR 标题需以 feat(): , docs(): , fix(): , style(): , refactor(): , chore(): 其中之一开头，例如：feat(component): 新增功能。
⚠️ The PR title must start with feat(): , docs(): , fix(): , style(): , or refactor(): , chore(): . For example: feat(component): add new feature.

如果跨多个组件，请使用主要组件作为前缀，并在标题中枚举、描述中说明。
If it spans multiple components, use the main component as the prefix and enumerate in the title, describe in the body.

如果是破坏性变更，请在类型后添加 !，例如 feat(component)!: 破坏性变更。
For breaking changes, add ! after the type, e.g., feat(component)!: breaking change.