perf(csvpp,csvpputil): cache reflection type info and optimize YAML encoding

[osamingo]

Summary

• Cache reflection type info (sync.Map) in Marshal/Unmarshal following encoding/json pattern
• Optimize YAML batch encoding with direct slice construction
• Add WithYAMLCapacity option for pre-allocating internal buffer

Changes

csvpp package (reflection cache)

• New file type_cache.go: typeInfo struct with sync.Map cache keyed by reflect.Type
• marshal.go: Refactor buildFieldMap, encodeRecord, MarshalWriter to use cached type info. Remove redundant buildHeaders
• export_test.go: Export CachedTypeInfo for testing
• benchmark_test.go: Add 100-record Marshal/Unmarshal benchmarks

csvpputil package (YAML optimization)

• yaml_array_writer.go: Add YAMLArrayWriterOption, WithYAMLCapacity, and encodeYAMLRecords for direct batch encoding in MarshalYAML/WriteYAML
• yaml_array_writer_test.go: Add capacity hint tests
• benchmark_test.go: Add WriteWithCapacity benchmark

Benchmark Results

Marshal/Unmarshal (reflection cache)

                  │   old    │           new            │
                  │  sec/op  │  sec/op    vs base       │
Marshal-12          1.810µ     1.173µ    -35.18%
Unmarshal-12        2.170µ     1.987µ     -8.44%

                  │   old    │           new            │
                  │ allocs   │  allocs   vs base        │
Marshal-12          36.00      30.00     -16.67%

YAML batch encoding

                  │    old     │          new            │
                  │   B/op     │   B/op    vs base       │
MarshalYAML-12      11.85Mi     11.82Mi   -0.28%
WriteYAML-12        11.76Mi     11.72Mi   -0.28%

Test Plan

• GOEXPERIMENT=jsonv2 go test -race ./... all pass
• Benchmark comparison with benchstat
• Review public API backward compatibility (no changes)

🤖 Generated with Claude Code