perf(parquet): reuse DELTA byte-array scratch

[fallintoplace]

Summary

• Reuse []parquet.ByteArray scratch for spaced compaction in DELTA_LENGTH_BYTE_ARRAY.
• Reuse the same scratch pattern in DELTA_BYTE_ARRAY.
• Clear the scratch after encoding so input payload slices are not retained.
• Add offset, correctness, reuse, and benchmark coverage.

Benchmark

64K values with 16-byte values on an Apple M1 Pro, Go 1.26.3, 1 CPU, 250 ms per sample, 9 samples. Values are medians.

Encoder / validity	ns/op before	ns/op after	B/op before	B/op after	allocs/op before	allocs/op after
DELTA_LENGTH_BYTE_ARRAY / all valid	1,209,684	886,099	1,577,916	5,000	2,055	2,054
DELTA_LENGTH_BYTE_ARRAY / 50% null	1,049,068	692,072	1,575,636	2,696	1,031	1,030
DELTA_BYTE_ARRAY / all valid	2,618,095	1,957,687	1,633,022	57,918	4,111	4,109
DELTA_BYTE_ARRAY / 50% null	1,776,488	1,141,407	1,604,656	31,066	2,063	2,061

Checks

• go test ./...
• go test -race ./parquet/internal/encoding -count=1
• go vet ./parquet/internal/encoding
• git diff --check

No public API changes.