refactor(AvatarAppearance): improve thread-safety and performance

[jamenai]

Replace Dictionary with ConcurrentDictionary for attachments and add proper locking mechanisms for texture, wearables, and visual parameter fields to prevent race conditions in multi-threaded scenarios.

Key changes:

• Use ConcurrentDictionary<int, List> for m_attachments
• Add lock objects (m_textureLock, m_wearablesLock, m_visualParamsLock)
• Protect all field access with appropriate locks
• Optimize GetAttachments() to minimize lock duration
• Fix potential collection modification exception in DetachAttachment()
• Use snapshots in Pack()/Unpack() to avoid holding locks during I/O

Benefits:

• Thread-safe read/write operations
• Reduced lock contention through fine-grained locking
• Better performance with concurrent dictionary for attachments
• No API changes - fully backward compatible

Refs: #performance #thread-safety