lang 2.4: parameterized-protocol method calls on pack elements

`xs[i].get()` on a parameterised `..xs: Box(T)` pack now resolves — the
canonical `ValueListenable` shape. registerParamImpl, for a CONCRETE-struct
source, now also registers the impl's methods as `<Source>.<method>` in
fn_ast_map (like a non-parameterised impl), so UFCS finds them. Such methods
are already fully concrete (`impl Box(s64) for IntCell` → `get(self: *IntCell)
-> s64`), so there's nothing to monomorphize; generic/pack sources stay lazy in
param_impl_map. First impl wins on a name collision.

Heterogeneous parameterised packs work: each `xs[i]` binds a different T and
dispatches to its own impl. Regression:
examples/194-protocol-pack-parameterized.sx (Box(s64) IntCell + Box(string)
StrCell, order-independent).

tests/expected/194-protocol-pack-parameterized.exit

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

tests/expected/194-protocol-pack-parameterized.txt

@@ -0,0 +1,2 @@
+first=11 second=hi
+first=x second=99