Enable multithreaded layout

As discussed by the original stretch project, the ability to multithread layout can be very useful.

We should have opt-in ways to do so.

No file, test, or entry point is named. Start by locating the layout execution path in the Taffy repository and reviewing the original Stretch project discussion referenced in the issue. Clarify the opt-in API, thread-safety expectations, and measurable completion criteria before attempting implementation.

Written by the indexing model from the issue text.