perf(table): borrow remaining DataFile metadata in files tables

[fallintoplace]

What

• Borrow ColumnSizes, KeyMetadata, SplitOffsets, and EqualityFieldIDs for built-in manifest DataFile values.
• Keep the public getter fallback for custom DataFile implementations.
• Add focused tests for both paths.
• Add a benchmark with empty, medium, and wide metadata.

Benchmark

Same benchmark fixture before and after. Apple M1 Pro, darwin/arm64, Go 1.26.3. -benchtime=3x.

• files=4096/medium/borrowed: 13.16 ms -> 8.67 ms, 17.3 MB -> 11.3 MB, 45,773 -> 17,102 allocs/op.
• files=16384/wide/borrowed: 181.0 ms -> 126.2 ms, 247.2 MB -> 153.5 MB, 197,465 -> 82,776 allocs/op.
• The public wrapper stayed at about 247.2 MB and 197K allocs/op in the wide case.

Command:

go test ./table -run '^$' -bench 'BenchmarkInspectContentFileAppenderDataFileCollections' -benchmem -benchtime=3x -count=1

Checks

• go test ./...
• go test -race ./table
• go test -tags=assert -run='^(TestInspectFilesTablesEarlyRelease|TestInspectDataFilesEmitsEmptyBatchWhenAllEntriesAreDeleted|TestInspectDataFilesEmptyTableEarlyRelease|TestInspectPositionDeletesEarlyRelease)$' ./table
• go vet ./...
• golangci-lint run --timeout=10m

[zeroshade]

LGTM — the borrowed metadata path preserves ownership and fallback semantics while materially reducing files-table allocations; tests and CI are green.

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