Rewrite state management with pure tree operations

[bartaxyz]

Summary

• Replace mutation-based tree operations with immutable pure functions using structural sharing
• Remove lodash and polished from production dependencies
• Reorganize src/ into components/, tree/, hooks/, types.ts
• Add benchmark suite for tree operations

Performance

Compared against the previous cloneDeep-based implementation:

Operation	50 nodes	584 nodes	4680 nodes
updateById (best case)	19.7x	32.1x	23.8x
updateById (worst case)	12.1x	22.9x	19.3x
setSelectedById	11.5x	12.6x	11.4x

What changed

• Tree operations (find, update, move, setSelected) are now pure functions that return new data instead of mutating the input array
• Structural sharing means only nodes on the path to the target are cloned
• triggerOnChange flag and selectedOnChange (dead code) removed
• onChange is called directly from each handler instead of through a deferred useEffect
• lodash removed (no more cloneDeep)
• polished removed (replaced with CSS color-mix())

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