mergePowerChannelLabel
fun mergePowerChannelLabel(existing: List<String>, channelIndex: Int, label: String): List<String>(source)
Merges a user-editable power-channel label into existing at the 0-based channelIndex.
Earlier channels keep their slot via blank padding, the label is trimmed, and trailing blanks are dropped so clearing a label shrinks the list. Shared by the DB write path and test fakes so the rule can't drift between them.