perf(arrow/ipc): release dictionary payloads eagerly

[fallintoplace]

What

• Release each dictionary payload as soon as WritePayload returns.
• Keep the optional dictionary delta slice in the same short-lived scope.
• This keeps encoded dictionary payloads from accumulating until writeDictionaryPayloads returns.

Benchmark

The benchmark uses a tracking allocator and reports peak bytes held by dictionary payloads beyond the input record.

Apple M1 Pro, Go 1.26.3, 1 CPU, 150 ms, 7 runs:

Dictionary columns	Before	After
8	1,536 B	192 B
64	12,288 B	192 B
256	49,152 B	192 B

Tests

• go test ./arrow/ipc
• PARQUET_TEST_DATA="$(pwd)/parquet-testing/data" go test ./...
• go vet ./arrow/ipc