perf(parquet): reuse DELTA_BYTE_ARRAY discard storage

[fallintoplace]

What

• Reuse decoder-owned storage while DELTA_BYTE_ARRAY values are discarded.
• Add correctness tests and discard benchmarks.

Why

• Discard currently allocates a new byte slice for every discarded value with a non-empty suffix.
• Skipping 65,536 values creates 65,535 allocations only to throw the values away.

Implementation

• Copy the first discarded value into owned scratch because the decoded suffix can alias page data.
• Grow the scratch buffer amortized and reuse it across pages.
• Keep empty suffixes on the existing zero-copy path.
• Benchmark prefix-heavy and low-prefix data at 1,024 and 65,536 values.

The benchmark changes from 65,535 allocations to 0 allocations per operation after warm-up, with about 3.3x lower discard time for 65,536 values.

Tests:

• go test ./parquet/internal/encoding -count=1
• go test ./parquet/file -run '^(TestWithEOFReader|TestInvalidHeaders|TestInvalidFooter|TestIncompleteMetadata|TestDeltaLengthByteArrayPackingWithNulls|TestDeltaBinaryPackedMultipleBatches|TestPageStreaming.*|TestPrimitiveReader|TestFullSeekRow|TestSkipEmptyRepeatedRows)$' -count=1
• go test -race ./parquet/internal/encoding -run 'TestDeltaByteArrayDecoder(DiscardsAllEmptyValues|DiscardCopiesFirstValue|ReusesDiscardScratch|RejectsInvalidPrefixes|KeepsPartialDecodeResults)$' -count=1
• go vet ./parquet/internal/encoding

[zeroshade]

discardScratch can escape through Decode and then be overwritten by a later Discard.

A concrete sequence is values ["aa", "aa", "zz"]: Discard(1) stores "aa" in the scratch; Decode(1) takes the empty-suffix path and returns a prefix backed by that scratch; the next Discard(1) reuses the scratch and changes the already-returned value to "zz".

I reproduced this on the current head: the retained decoded value mutated from "aa" to "zz". This breaks the decoder's existing result-lifetime invariant, including across SetData because the scratch is intentionally retained. Please ensure storage exposed by Decode cannot alias reusable discard scratch and add this Discard→Decode(empty suffix)→Discard transition as a regression case.