zipper,node: reduce wrapper churn and grow key scratch geometrically

[snissn]

Summary

• switch zipper loadNode to node.NewNodeView/value wrappers to avoid per-load *Node allocations
• thread value-node wrappers through recursive and coalesce leaf merge paths
• grow node.ensureKeyScratch geometrically to avoid per-step realloc churn
• add node unit coverage for scratch reuse growth behavior

Testing

• go test ./TreeDB/node ./TreeDB/zipper
• go test ./TreeDB/caching ./TreeDB/internal/memtable ./TreeDB/zipper ./TreeDB/node

[Copilot]

Pull request overview

Reduces allocation overhead in the zipper hot path by returning node.Node values instead of *node.Node pointers from loadNode, and grows the key scratch buffer geometrically to avoid per-step reallocations.

Changes:

• loadNode now returns node.Node (value) using NewNodeView, avoiding per-load heap allocations; callers take & when passing to functions expecting *node.Node
• ensureKeyScratch grows geometrically (doubling from min 64) instead of exact-fit allocation
• Added unit test for geometric scratch reuse behavior

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
TreeDB/zipper/zipper.go	loadNode returns value node.Node via NewNodeView; callers use & for pointer params
TreeDB/node/leaf.go	Geometric growth for keyScratch buffer
TreeDB/node/key_scratch_test.go	New test verifying scratch reuse and growth

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[snissn]

Closing in favor of standalone PR #825 (same core changes without stacked experimental dependencies).}