feat(poi): benchmark device/precision propagation and reliability fixes

[sainijit]

• Add --resource_config CLI arg to poi_stream_density.py so DEVICE and MODEL_PRECISION are propagated through all stream-density re-init iterations (make benchmark DEVICE=all-gpu.env now works end-to-end)
• Catch PermissionError in _save_alert_thumbnails: logs a clear warning instead of crashing when results/ is root-owned
• Clarify true E2E latency measurement: the primary VLM metrics-file latency (e.g. 128ms) is frame capture → alert dispatch (start set via user_log_start_time with DLStreamer frame timestamp, end via log_end_time at alert dispatch). Corrects misleading comments that said the metric excluded DLStreamer pipeline latency.

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• Every commit is a single defect fix and does not mix feature addition or changes
• Unit Tests have been added for new changes
• Updated Documentation as relevant to the changes
• All commented code has been removed
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• PR change contains code related to security
• PR introduces changes that breaks compatibility with other modules (If YES, please provide details below)

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