feat(server): soft-close thinking termination (qwen35 + gemma4) #1101

Job	Run time
uv workspace (lock + sync + import smoke)	1m 6s
Build (cmake + uv sync --extra megakernel)	18m 14s
GPU tests (self-hosted RTX 3090, sm_86)	7s
GPU tests (self-hosted Radeon 8060S, gfx1151 / ROCm)	8s
	19m 35s

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