[felixge]

Summary

Related to #40.

• Nest the writer and hook fields in a shared, value-embedded rwFields type, so all 512 wrapper variants describe one embedded field instead of repeating 14 field descriptors.
• Preserve wrapper methods, exact interface sets, hook initialization, memory layout, and the single state allocation.
• Add a layout regression test and a reproducible release/local binary-size comparison experiment.

This is a metadata-only improvement. It does not include method promotion or metrics-specific initialization, and does not eliminate the full size increase reported in #40.

Binary-size results

Same HTTP server before/after, darwin/arm64, CGO disabled, with identical module replacement and build settings:

Toolchain	Default bytes saved	Stripped bytes saved
Go 1.26.7	232,144 (2.35%)	232,192 (3.39%)
Go 1.27.1	232,160 (2.30%)	232,160 (3.33%)

Machine-code section sizes and allocation counts are unchanged. Full reproduction instructions and measurements are in experiments/binary-size/README.md.

Validation

• go test ./... on Go 1.26.7 and Go 1.27.1, including all 512 interface combinations.
• go test -race ./... and go vet ./... on Go 1.27.1.
• Allocation benchmarks: Wrap remains 128 B/1 allocation; CaptureMetrics remains 417 B/13 allocations.
• Regeneration consistency, shell syntax, and diff checks.