ffi M5.A.next.2b.fu2.A: generic \$R pack-fn — lock in silent-zero return

Follow-up #2 from step 2b: pack-fns with a generic return type
(`(..\$args) -> \$R`). Today's `monomorphizePackFn` calls
`resolveReturnType` which sees `\$R` as a generic name and
returns an opaque struct TypeId. The mono's ret_ty is wrong
and the value silently coerces to 0.

`examples/159-pack-generic-ret.sx` pins this: `first(42)` and
`first(99)` both return `0` instead of the call arg. The lock-in
captures the wrong output as the snapshot to flip.

Next commit infers the ret type from the body's tail expression
(arrow form) or the first explicit `return X;` (block form),
then builds the mono signature against that concrete type.

198/198 example tests + \`zig build test\` green.

tests/expected/159-pack-generic-ret.exit

@@ -0,0 +1 @@
+0

tests/expected/159-pack-generic-ret.txt

@@ -0,0 +1 @@
+0 0