emit-fn lowered multi-arity fns to a Scheme case-lambda and variadic fns to
a rest-arg lambda; the Scheme rest list is coerced to a jolt seq (nil when
empty, via list->cseq), and the named-let wrapper runs that coercion only on
first entry so recur carries the seq directly. Single fixed arity keeps the
plain-lambda fast path (fib untouched).
Also fixes a latent leak in the module-global known-procs: a throw mid-emit
(uncompilable body) left the fn's name registered, so a later corpus case
binding the same name to a keyword emitted a direct call to a non-procedure.
The cleanup now runs on the error path too. Only surfaced once the new arity
support let +24 cases compile further before hitting an uncompilable fn.
Gate: emit-test 81/81, subset probe 619/619 compiled (was 595), 0 divergences,
2036/2655 out of subset; full run-tests green (125 files).
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