Polish debug chunk source generator

[mooskagh]

Summary

• add a DebugChunkSourceGenerator stage that emits deterministic DebugChunkSource instances for loader bring-up
• extend DataLoaderConfig and the stage factory to recognize the debug generator and wire it into the build
• add a focused unit test and register it with Meson alongside the existing suite
• switch the generator shuffle and pacing logic to Abseil utilities to satisfy repository guidance
• polish the debug generator lifecycle, queue handling, and test so the implementation stays concise and idiomatic

Testing

• just pre-commit

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https://chatgpt.com/codex/tasks/task_e_68e101e81f748331a689058f164ee77d

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Prevent Stop() from hanging on full output queue

Stop() only calls request_stop() and joins the thread, while the worker loop writes via blocking producer.Put (line 92) and never closes the queue. If a downstream stage has stopped and the queue fills, the worker blocks inside Put and can’t observe the stop token, causing Stop() to block indefinitely and preventing the loader from shutting down. Closing the queue or the producer before joining would let the worker unblock even when no consumer is draining.

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