ir: resolveTypeArg failure paths return .unresolved, not .void

The three post-diagnostic failure returns in resolveTypeArg (pack-index OOB,
no active pack binding, unresolved type name) returned .void as a sentinel.
Per the CLAUDE.md rule (.void is unacceptable for a failed type lookup -- it
conflates with the real void type), use the dedicated .unresolved sentinel.
They follow addFmt(.err) so compilation aborts before codegen; behavior is
unchanged, the sentinel is now correct. 236 + unit green.

src/ir/lower.zig

@@ -9815,7 +9815,7 @@ pub const Lowering = struct {
                             if (arg_tys.len == 1) @as([]const u8, "") else @as([]const u8, "s"),
                         });
                     }
-                    return .void;
+                    return .unresolved;
                 }
             }
             if (self.diagnostics) |diags| {
@@ -9823,7 +9823,7 @@ pub const Lowering = struct {
                     pi.pack_name, pi.index,
                 });
             }
-            return .void;
+            return .unresolved;
         }
         // Bare `$<name>` in a type-arg position. Single-type generic
         // bindings (`$R: Type` in `Closure(..$args) -> $R`) live in
@@ -9849,7 +9849,7 @@ pub const Lowering = struct {
                 if (self.diagnostics) |diags| {
                     diags.addFmt(.err, node.span, "unresolved type: '{s}'", .{id.name});
                 }
-                return .void;
+                return .unresolved;
             },
             .type_expr => |te| {
                 if (self.type_alias_map.get(te.name)) |alias_ty| return alias_ty;