lang: inline for element form over packs — multi-iterable parity

src/ir/lower.zig

@@ -129,6 +129,12 @@ pub const Binding = struct {
     ty: TypeId,
     is_alloca: bool, // true if ref is a pointer that needs load
     is_ref_capture: bool = false, // `for xs: (*x)` — `ref` is `*elem`; auto-deref in value positions
+    /// `inline for xs (x)` element capture: `x` is an AST ALIAS for the
+    /// synthesized `xs[<i>]` of the current unroll iteration (`ref` is
+    /// `.none`). Identifier consumers substitute this node, so the capture
+    /// inherits the full pack-element semantics — concrete-arg substitution,
+    /// typing, and the interface-only constraint check.
+    pack_elem: ?*ast.Node = null,
 };
 
 // `init` / `deinit` / `put` are pub so collaborator unit tests (e.g.