Implement job.result_chunk event kind

[nficano]

ARCP v1.1 §8.4 introduces result_chunk event for streamed results. Not in PHP SDK.

Steps:

1. Add ResultChunk message in src/Messages/Job/
2. Register @SerialName('result_chunk') (or PHP equivalent attribute)
3. Implement client-side stream assembler concatenating chunks until final: true
4. Add samples/result_chunk/ demo
5. Tests

Reference: spec §8.4.

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Audit reference: ARCP SDK Audit v1.1 (2026-05-19)