Restore validated CUDA forward fixes on tts-arena/main

[shsym]

Restores the validated CUDA/runtime correctness chain on the canonical release branch: safe prefill-only execution, per-forward logits behavior, actionable async errors, fail-closed response status, recurrent no-sampler handling, and dense-logit prefill chunking. This intentionally omits unrelated branch commits and preserves focused regression coverage.

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