For my (buggy, unfinished, languishing without updates) prototype Ruby compiler, lambda and proc (and blocks) are all implemented nearly the same way, with the exception that for proc/blocks, return/break/next will act as if having unwound the stack to the scope where the proc was defined first (or throw an error if escaped from there).
The distinction is very obvious when you think of it in that way - a proc acts as if called in the context/scope it was defined, while a lambda acts as if called in the scope it is called in.
> How do you as a method author opt in to distinguishing between the two after yielding to a block?
You don't. A block acts as a proc, not a lambda. If you want lambda semantics, then take a lambda as an argument - don't take a block/proc and be surprised it doesn't act like something it isn't.