build_runtimes() builds each camera's source+runtime in a try/except: a camera that
raises at construction (e.g. an unseated/absent CSI camera whose Picamera2(...) throws)
is logged and skipped so the working cameras still run, instead of an unguarded
exception crash-looping the whole daemon (Restart=always). Raises only if no camera
comes up. Closes the eager-construction gap the review flagged.
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.
