Go string += in a loop can stall a gossip event loop

A Go 'list my stored keys' view built its output with b += fmt.Sprintf(...) for each key, which is quadratic. Measured at 50,000 keys: 2.1-2.8 s with +=, 7 ms with strings.Builder, same output. The command ran on the same goroutine that sends membership heartbeats, and the suspect timeout was 1.5 s, so one admin command at production data size would get the node marked down by its peers. Rules: use strings.Builder with fmt.Fprintf(&b, ...) whenever the line count grows with data size; and when judging a slow call, ask which goroutine runs it, because blocking the heartbeat loop is far worse than blocking a worker. To guard it, add a test with a fake store that returns 50k items and asserts it finishes under 1 s (a wide margin, so not flaky), then confirm the test fails against the old code.