Readonlysignal: AbortSignalAborts when the WebContents that initiated this invocation is destroyed.
Shared by every invocation from the same sender, and built on first read:
reading it needs a global AbortController, so Electron 15 or newer.
In a stream callback it is per invocation instead, and also aborts when
the renderer cancels that stream.
IpcMainInvokeEvent(forhandle/handleOnce) with a typedsenderand lifecycle signal.