[chmp]

Summary

Various performance experiments.

• avoids missing-field scans for complete structs;
• compacts static field-name caching;
• remember canonical derived-struct layouts and skips repeated lookup/seen bookkeeping with safe fallback;
• preserves duplicate field-name map semantics while allowing early lookup exit for unique schemas.
• batches List and Binary offset updates;
• uses append-oriented validity bitmap writes;
• reserves BinaryView backing bytes for known-length sequences.
• avoids discarded root metadata-vector allocation;
• preallocates struct/union conversion and wide-schema setup vectors.

Checklist

• Run the pre-commit checks locally with uv run python x.py precommit
• Add tests for any new functionality
• Update the relevant changelog (Changes.md for serde_arrow, marrow/Changes.md for marrow)

Benchmarks

func	benchmark	main	mr	change
to_marrow	complex_1000	4.86	2.67	-45%
to_marrow	primitives_1000	9.74	3.99	-59%
to_arrow	complex_1000	4.82	2.96	-39%
to_arrow	primitives_1000	9.67	6.65	-31%