Function background_event_key 

fn background_event_key(event: &Value) -> String

Extract a coalescing key from a background-lane event. Background events are RPC-wrapped MPS events: { “eventtype”: “rpc”, “data”: { “command”: “eventrecv”, “data”: { “event”: “sysinfo”, “scopes”: [“local”] } } } Key = “:”, e.g. “sysinfo:local” or “blockstats:block:abc123”. Falls back to “:” for events that don’t match the expected shape (safe: they coalesce to one slot, which is acceptable because non-sysinfo/blockstats never reach this lane).