Good writeup. The Arguments arc (#7→#13) hits close — did basically the same dance for an async step evaluator in Rust a while back. Went all in on Cow<'_, Input> assuming borrow-in-the-common-case would earn its keep. Microbenches looked great. Real workload: the Cow discriminant plus lifetime gunk bled into every combinator past the first await, inlining fell off a cliff, the whole point of Cow evaporated. Ripped it out for NoInput / OneInput<T> / MultiInput(Vec<T>) at the evaluator boundary — same split as your ZeroArguments / OneArgument / TwoArguments, just arrived at the ugly way.
One thing I keep wondering: have you stacked arity specialization with type specialization on the native path? Binary<add, int, int> style, drops the isInt probe altogether. Guessing the code size math didn't work out, or ICs are already soaking up whatever's hot on the object side so the native fast paths don't matter much. Which one?