v1.5.21

• Infer generic parameters nested inside metatypes and preserve those parameters through substitution.
• Select a unique most-specific fixed-arity overload for contextual typing while retaining safe union fallback for ambiguous, incomparable, and variadic candidates.
• Preserve the refined return type of assert-style helpers when their truthy success effect matches the normal return value.
• Narrow optional and falsy return expansions structurally, including generic instantiations and channel-select flows, without introducing recursive-type loops.
• Add order-independence, ambiguity, variadic, assertion, generic, and channel regression coverage.