run_loop now accepts a single runtime or a list; it steps every camera each tick
(one shared clock read), isolates a per-camera step exception, runs the periodic
sync/heartbeat/flush once, and routes zones per camera via a zones_for(cfg, rt)
callback. parse_cameras() reads CAMERAS='backend:device:camera_id,...' (single-camera
fallback when unset) and refuses to yield zero cameras. zones_from_config() gains a
camera_id filter; PipelineRuntime exposes camera_id. Single-camera behavior unchanged.
