Update clipboardchange event explainer with changeId, sticky activation, and partition-based behavior.

[roraja]

Key Changes

🆕 New Features

• changeId property: Added BigInt identifier for multi-window event deduplication
• Partition-based behavior: ChangeId generation now partition-specific for enhanced privacy
• Enhanced IDL: Updated interface with ClipboardChangeEventInit dictionary and constructor

🔒 Security Improvements

• Sticky activation requirement: Events now require user interaction (with persistent permission exception)
• Cross-partition isolation: Prevents correlation across partition boundaries
• Focus requirement: Clarified system focus terminology and behavior

📝 Documentation Enhancements

• Multi-window deduplication: Added comprehensive ClipboardManager example with memory leak prevention
• Updated code samples: Demonstrated changeId usage and duplicate event handling
• Partition privacy model: Added section explaining partition-based privacy benefits
• Alternative considerations: Updated rationale for sticky activation over no-activation approach

🔧 Technical Updates

• Updated IDL definitions with changeId as bigint
• Enhanced JavaScript examples with practical deduplication patterns
• Improved clipboard reading guidance with activation context
• Added server sync patterns for multi-device scenarios

Motivation

These changes strengthen the privacy and security model while enabling better multi-window application support. The partition-based approach aligns with modern web platform privacy principles, and the changeId property solves real-world deduplication challenges for applications with multiple tabs/windows.