`.filter(fn)` required the callback to return exactly bool. JS applies
ToBoolean to whatever the predicate answers, so `xs.filter((s) => s)` — the
idiomatic drop-the-falsy — was refused for no semantic reason.
The filter loop now wraps the call in the same toBool an `if` statement would
apply. A bool answer is unchanged and emits identical IR. A union answer
routes through its interned per-arm truthy helper, requested at the call site
where a real node exists for the diagnostic. A unit-only answer is constantly
falsy, and the call still runs for its effects.
The kinds with no native ToBoolean keep the fence: void has no value at all,
and dyn/jsval/caught truthiness needs the embedded engine.
Verified against Node: filtering strings, numbers, and a
`(string | undefined)[]` all print identically in both.
Diagnostics 100, filter corpus 9 across both
backends.
Jimmy Miller