improve "watch --save-data" output: include all timeout values

In the places where we use the output from --save-data narrow_timeouts to provide a clear error message, we would need to have not only the timeout that was used to declare that a unit has a narrow timeout, but also to have the original timeout and the margin factor.

This MR adds this information in the JSON output of --save-data.

Some refactoring was needed to preserve the narrow timeout margin parameter along the way.

(written with Claude Code, Opus 4.6)