perf(parquet): eliminate per-value allocation in delta bit-pack decoder

[zeroshade]

Rationale for this change

The delta bit-pack decoder's unpackNextMini() method calls BitReader.GetValue() once per value in each miniblock. GetValue() allocates a fresh []uint64 slice on every call.

For the default block size of 128 with 4 miniblocks of 32 values each, this causes ~128 heap allocations per block, or ~2048 allocations per 1024-value page. This allocation pressure dominates the decoder's runtime and generates significant GC load.

What changes are included in this PR?

1. Reused buffer: Added a deltaBuf []uint64 field to the decoder struct that is allocated once and reused across calls, eliminating the per-value allocations.
2. Width=0 fast-path: When deltaBitWidth == 0 (all deltas are identical, common for sequential or constant data), skip the bit reading entirely and directly accumulate minDelta

Are these changes tested?

Yes, the existing test suite passes along with all encoding and property tests

Are there any user-facing changes?

No user-facing API changes, purely an internal optimization

Benchmark Results (darwin/arm64, Apple M4, Go 1.25)

Baseline (main):

goos: darwin
goarch: arm64
pkg: github.com/apache/arrow-go/v18/parquet/internal/encoding
cpu: Apple M4
BenchmarkDeltaBinaryPackedDecodingInt32/len_1024-10     101904   11393 ns/op    359.52 MB/s   4912 B/op   2 allocs/op
BenchmarkDeltaBinaryPackedDecodingInt32/len_1024-10     107488   11192 ns/op    365.97 MB/s   4912 B/op   2 allocs/op
BenchmarkDeltaBinaryPackedDecodingInt32/len_1024-10     107224   11338 ns/op    361.26 MB/s   4912 B/op   2 allocs/op
BenchmarkDeltaBinaryPackedDecodingInt32/len_1024-10     105836   11338 ns/op    361.26 MB/s   4912 B/op   2 allocs/op
BenchmarkDeltaBinaryPackedDecodingInt32/len_1024-10     105051   11167 ns/op    366.78 MB/s   4912 B/op   2 allocs/op
BenchmarkDeltaBinaryPackedDecodingInt32/len_65536-10      1846  653185 ns/op    401.33 MB/s   4912 B/op   2 allocs/op
BenchmarkDeltaBinaryPackedDecodingInt32/len_65536-10      1914  631663 ns/op    415.01 MB/s   4912 B/op   2 allocs/op
BenchmarkDeltaBinaryPackedDecodingInt32/len_65536-10      1912  630612 ns/op    415.70 MB/s   4912 B/op   2 allocs/op
BenchmarkDeltaBinaryPackedDecodingInt32/len_65536-10      1916  633029 ns/op    414.11 MB/s   4912 B/op   2 allocs/op
BenchmarkDeltaBinaryPackedDecodingInt32/len_65536-10      1939  640181 ns/op    409.48 MB/s   4912 B/op   2 allocs/op

Optimized (this PR):

goos: darwin
goarch: arm64
pkg: github.com/apache/arrow-go/v18/parquet/internal/encoding
cpu: Apple M4
BenchmarkDeltaBinaryPackedDecodingInt32/len_1024-10     435112    2768 ns/op   1479.84 MB/s   4912 B/op   2 allocs/op
BenchmarkDeltaBinaryPackedDecodingInt32/len_1024-10     434569    2768 ns/op   1479.95 MB/s   4912 B/op   2 allocs/op
BenchmarkDeltaBinaryPackedDecodingInt32/len_1024-10     434536    2772 ns/op   1477.51 MB/s   4912 B/op   2 allocs/op
BenchmarkDeltaBinaryPackedDecodingInt32/len_1024-10     411572    2859 ns/op   1432.55 MB/s   4912 B/op   2 allocs/op
BenchmarkDeltaBinaryPackedDecodingInt32/len_1024-10     419126    2862 ns/op   1431.03 MB/s   4912 B/op   2 allocs/op
BenchmarkDeltaBinaryPackedDecodingInt32/len_65536-10      8756  136708 ns/op   1917.55 MB/s   4912 B/op   2 allocs/op
BenchmarkDeltaBinaryPackedDecodingInt32/len_65536-10      8850  136880 ns/op   1915.14 MB/s   4912 B/op   2 allocs/op
BenchmarkDeltaBinaryPackedDecodingInt32/len_65536-10      8913  136911 ns/op   1914.70 MB/s   4912 B/op   2 allocs/op
BenchmarkDeltaBinaryPackedDecodingInt32/len_65536-10      8158  145797 ns/op   1798.01 MB/s   4912 B/op   2 allocs/op
BenchmarkDeltaBinaryPackedDecodingInt32/len_65536-10      8281  142564 ns/op   1838.79 MB/s   4912 B/op   2 allocs/op

Summary:

Size	Baseline (ns/op)	Optimized (ns/op)	Speedup	Baseline (MB/s)	Optimized (MB/s)
1024	11,286	2,806	4.0x	363	1,460
65536	637,734	139,772	4.6x	411	1,877

• ~4-4.6x faster decoding
• Zero additional allocations (2 allocs/op unchanged — those are from test setup)
• Encoding performance is unchanged (encoder path not modified)