perf(parquet): reuse dictionary RLE decoder across pages

[fallintoplace]

Summary

• Reuse the dictionary RLE decoder between data pages.
• Reset the existing TypedRleDecoder and its bytes.Reader in SetData.
• Keep the current bit-width validation and empty-page behavior.
• Add a page-transition regression test covering partial runs, literal runs, and empty data.
• Add small-page and SetData allocation benchmarks.

Benchmark

Local Apple M1 Pro run with GOMAXPROCS=1: repeated SetData improved from about 0.6-1.0 us / 4.9 KiB / 3 allocs to 6.3 ns / 0 B / 0 allocs.

Tests

• go test ./parquet/internal/encoding ./parquet/internal/utils
• go test -race ./parquet/internal/encoding ./parquet/internal/utils
• go vet -composites=false ./parquet/internal/encoding ./parquet/internal/utils
• GOOS=linux GOARCH=386 go test -c -o /dev/null ./parquet/internal/encoding

User-facing changes

No.