Performance: Optimize TreeWalker usage in selection handling

[devin-ai-integration]

Performance Improvements and onBeforeInput Edge Case Implementation

Summary

This PR implements two key improvements to the edytor text editor:

1. TreeWalker Optimization: Added WeakMap-based caching for TreeWalker instances in selection handling to reduce DOM API overhead during frequent selection operations.

2. onBeforeInput Edge Case: Completed the missing implementation for block-spanning backward deletion scenarios, adding proper post-deletion handling for nested blocks and empty block cleanup.

Both changes follow existing code patterns and are designed to improve performance and feature completeness without breaking existing functionality.

Review & Testing Checklist for Human

• Test TreeWalker optimization with various selection scenarios - Navigate through text, select across blocks, verify cursor positioning works correctly and performance feels smooth
• Test block-spanning backward deletion edge cases - Select text across multiple blocks and press backspace, especially with nested blocks and empty blocks
• Monitor memory usage during extended editing sessions - Open dev tools memory tab and edit for several minutes to ensure no memory leaks from TreeWalker caching
• Verify selection positioning after complex deletions - After block-spanning deletions, ensure cursor ends up in the correct position and text content is as expected
• Test nested block scenarios specifically - Create nested block structures and test backward deletion to ensure proper unnesting behavior

Note: CI failures are deployment infrastructure-related (Cloudflare Pages, Netlify), not code issues. Local build and tests pass except for a pre-existing setBlock test failure that exists on master branch.

---

Diagram

%%{ init : { "theme" : "default" }}%%
graph TD
    A["src/lib/selection/selection.svelte.ts"]:::major-edit --> B["TreeWalker Cache"]
    A --> C["findTextNode()"]
    A --> D["setAtTextRange()"]
    
    E["src/lib/events/onBeforeInput.ts"]:::major-edit --> F["deleteContentBackward case"]
    E --> G["Block-spanning deletion"]
    
    H["src/lib/edytor.utils.ts"]:::context --> I["deleteContentWithinSelection()"]
    I --> F
    
    J["src/lib/block/block.svelte.ts"]:::context --> K["unNestBlock()"]
    J --> L["mergeBlockBackward()"]
    K --> F
    L --> F
    
    M["PERFORMANCE_ANALYSIS.md"]:::minor-edit --> N["Performance Report"]
    
    subgraph Legend
        L1["Major Edit"]:::major-edit
        L2["Minor Edit"]:::minor-edit  
        L3["Context/No Edit"]:::context
    end
    
    classDef major-edit fill:#90EE90
    classDef minor-edit fill:#87CEEB
    classDef context fill:#FFFFFF

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Notes

• Performance Impact: The TreeWalker optimization is expected to reduce selection operation overhead by 30-50% based on the analysis in PERFORMANCE_ANALYSIS.md
• Memory Safety: WeakMap automatically handles cleanup when DOM nodes are garbage collected, preventing memory leaks during long editing sessions
• Code Patterns: Both implementations follow existing patterns in the codebase for consistency
• Session Info: Requested by @beynar - Link to Devin run: https://app.devin.ai/sessions/d1bc67f55bc34d16922ed61eae9dd0d2

Pre-existing Issue: There's a failing test in setBlock.test.tsx showing text duplication ("Hello world!Hello world!") that exists on both this branch and master - this appears unrelated to the current changes and should be addressed separately.

[devin-ai-integration]

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