zipper: pool recursive root builders in write path

[snissn]

Summary

• pool root builders in writeRecursive for both leaf and internal paths
• re-use existing builder pool/reset flow already used by split builders
• keep builder scratch release semantics unchanged (ReleaseScratch before re-pool)

Why

batch_write_steady alloc profiles still showed zipper write-path wrapper churn, including node.NewBuilderWithOptions under zipper.(*Zipper).writeRecursive.

Validation

• go test ./TreeDB/zipper ./TreeDB/caching
• time ./bin/unified-bench -dbs treedb -profile fast -keys 500000 -progress=false -treedb-index-outer-leaves-in-vlog=true -valsize=100 -treedb-force-value-pointers=false -checkpoint-between-tests -profile-dir=/home/mikers/tmp/perf-zipper-builderpool-b-1773435791

Comparison baseline (PR #819 head): /home/mikers/tmp/perf-batch-geogrow-b-1773435448

alloc_space (batch_write_steady)

• total: 122.66MB -> 111MB
• zipper.(*Zipper).writeRecursive: 56.22MB -> 53.71MB
• node.NewBuilderWithOptions: 8.50MB -> ~0MB

ops/sec (selected)

• batch_write_steady: 2,231,000 -> 2,239,040 (+0.4%)
• batch_write: 12,610,767 -> 12,496,662 (-0.9%)
• batch_random: 8,278,773 -> 8,207,744 (-0.9%)
• dataset_write_sorted: 3,634,457 -> 3,726,001 (+2.5%)

[Copilot]

Pull request overview

Refactors Zipper’s page-builder allocation to reuse node.Builder instances via a shared pool, reducing per-merge allocations on the write path.

Changes:

• Introduces newPooledBuilderForType / releasePooledBuilder to pool builders for both leaf and internal pages.
• Switches writeRecursive to use pooled builders for leaf and internal merges (with deferred release).
• Keeps releasePooledLeafBuilder as a thin wrapper for existing call sites.

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[Copilot]

Pull request overview

This PR refactors Zipper’s page-builder allocation to use pooled node.Builder instances more broadly, reducing per-merge allocations during recursive writes.

Changes:

• Simplified newPooledLeafBuilder by delegating to a new generalized newPooledBuilderForType.
• Introduced releasePooledBuilder and updated call sites to return builders to the pool in writeRecursive.
• Added support for pooled builder creation for internal pages (including InternalBaseDelta option propagation).

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