perf(query-planner): reduce allocations and hash-based node comparison in query planner

[tninesling]

Node comparisons:

• When comparing potential merge candidate in can_merge_sibling_in, this change compares hashes of the nodes instead of a deep data comparison. Note that this does not affect the check for merging grandchild nodes, as that caused the planner to miss merge opportunities in a previous commit

Allocation savings:

• Converts subgraph_and_merge_at_key from string formatting to numeric hash
• .iter() instead of .clone().into_iter() on possible_types
• .copied() instead of .cloned() on Copy-type NodeIndex
• Move owned Vec instead of cloning in flat_wrap_nodes

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