Add Layers panel support for displaying multiple groups with instances of the same children layers

[Keavon]

Closes #2153

[gemini-code-assist]

Code Review

This pull request refactors the layer panel to support layer instances by using instance paths (sequences of node IDs) instead of single node IDs. This allows the same layer to appear in multiple locations within the hierarchy while maintaining independent expansion states. Key changes include a rewritten graph traversal for building the layer structure, updated serialization for backward compatibility, and frontend adjustments to handle path-based expansion and selection. A suggestion was made to use a more idiomatic filtering approach when extending the list of collapsed layers during recursive operations.

[cubic-dev-ai]

No issues found across 6 files

Confidence score: 5/5

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