[codex] batch leaf log split appends

[snissn]

Summary

Shortens the leaf-log append critical path for split sibling leaves without changing leaf-generation liveness semantics.

Changes:

• adds a LeafPageBatchLog path in the zipper
• implements AppendLeafPages for the caching leaf-page log so sibling leaf pages can be compacted before entering the value-log append path and persisted through one append call
• switches the two-leaf split path to persist both sibling leaves together
• forces outer-leaf value-log records to stay one leaf per frame, preserving record-granular leaf-generation accounting

Correctness note

I intentionally did not batch bulk-build leaf generation in this PR. A broader batching attempt caused TestLeafGenerationPack_RewritesCollectionInternalRootsBeforeGC to fail because dead-byte accounting no longer saw the expected dead leaf records. This PR keeps the safe split-sibling batching only and keeps outer-leaf frames at one leaf record per frame.

Performance evidence

Correctness guard that specifically protects leaf-generation accounting:

GOWORK=off go test ./TreeDB/db -run '^TestLeafGenerationPack_RewritesCollectionInternalRootsBeforeGC$'

Result:

ok  github.com/snissn/gomap/TreeDB/db  (cached)

Focused package tests:

GOWORK=off go test ./TreeDB/caching ./TreeDB/zipper ./TreeDB/db \
  -run 'TestCachingLeafPageLog|TestZipper|TestLeafPageLog|TestLeafGenerationPack_RewritesCollectionInternalRootsBeforeGC'

Result:

ok  github.com/snissn/gomap/TreeDB/caching 0.050s
ok  github.com/snissn/gomap/TreeDB/zipper 0.004s
ok  github.com/snissn/gomap/TreeDB/db 0.143s

Example update benchmark run on this branch:

GOWORK=off go test ./cmd/mongo_gateway_bench \
  -run '^$' \
  -bench 'BenchmarkDirectCollectionConcurrentUpdateBSONIndexes2$' \
  -benchmem \
  -benchtime=1s \
  -count=1

Selected result line:

BenchmarkDirectCollectionConcurrentUpdateBSONIndexes2-12  167571  6299 ns/op  158747 docs/sec  1455 B/op  3 allocs/op

Leaf-log/root-apply metrics from the same run:

publish_delta_group_root_apply_ns/doc=476.2
publish_delta_group_root_apply_leaf_log_pages_written/doc=0.09181
publish_delta_group_root_apply_leaf_log_record_hint_write_bytes/doc=61.12
publish_delta_group_lock_hold_ns/doc=3.792
update_buffer_lock_hold_ns/doc=525.1

Full local benchmark output was saved under /tmp/gomap_leaflog_pr_validation_* on the development host.