Implement job.result_chunk event kind

[nficano]

ARCP v1.1 §8.4 introduces a result_chunk event kind for streamed result output. Not implemented (kotlin uses generic stream.chunk instead).

Steps:

1. Add JobResultChunk data class in messages/Execution.kt with the v1.1 payload (sequence, data, final)
2. Register @SerialName("result_chunk") event kind
3. Add to the RESERVED_EVENT_KINDS set
4. Implement client-side stream assembler that concatenates chunks until final: true
5. Add samples/ResultChunk/ mirroring typescript-sdk/examples/result-chunk/
6. Add tests

Reference: spec §8.4, §13.6.

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