perf(arrow/array): append dictionary indices directly

[fallintoplace]

What does this change?

• DictionaryBuilder.AppendIndices used to allocate a new typed slice on every call.
• Convert the input values directly into the final typed index buffer.
• Keep the existing bulk validity bitmap path.
• Add coverage for all eight index types, nullable appends, split appends, and invalid validity lengths.
• No public API changes.

Benchmark

Command:

go test ./arrow/array -run '^$' -bench '^BenchmarkDictionaryBuilderAppendIndices$' -benchmem -benchtime=300ms -count=8

Apple M1 Pro, Go 1.26.3. The benchmark uses 65,536 indices and a 256-entry dictionary.

Case	Before ns/op	After ns/op	Before B/op	After B/op	Allocs
int32, non-null	110,370	75,398	815,908	553,762	18 -> 17
int32, nullable	116,578	75,895	815,907	553,762	18 -> 17

This is about 32% less time and allocation bytes for the non-null case, and about 35% less time for the nullable case.

The Parquet decoder's separate []uint64 -> []int scratch conversion is intentionally outside this PR.

Tests

• go test ./...
• go test -race ./arrow/array
• go vet ./arrow/array