ir: resolveAstType pack-index arm returns .unresolved, not .s64

The pack-aware caller (resolveTypeWithBindings) resolves pack-index type
exprs against the active binding before delegating, so reaching this bare
type_bridge path means the binding was missing. .s64 silently fabricated
an 8-byte int; return the .unresolved sentinel so it surfaces (trips the
sizeOf/toLLVMType panic at codegen). Closes the last .s64 escape in
resolveAstType.

src/ir/type_bridge.zig

@@ -34,11 +34,14 @@ pub fn resolveAstType(node: ?*const Node, table: *TypeTable) TypeId {
         .pack_index_type_expr => {
             // Pack-index `$args[N]` in a type position must be resolved
             // against an active pack binding — `type_bridge` has no access
-            // to that state, so this bare path falls back to .s64 with a
-            // diagnostic logged. The pack-aware caller (lowering's
-            // `resolveTypeWithBindings`) handles this case directly.
-            std.debug.print("type_bridge: pack-index type expression encountered outside a pack-aware context\n", .{});
-            return .s64;
+            // to that state. The pack-aware caller (lowering's
+            // `resolveTypeWithBindings`) handles this case directly *before*
+            // delegating here, so reaching this bare path means the binding
+            // was missing. `.s64` would silently fabricate an 8-byte int;
+            // return `.unresolved` so it surfaces (trips the sizeOf/toLLVMType
+            // panic at codegen).
+            std.debug.print("type_bridge: pack-index type expression encountered outside a pack-aware context — returning .unresolved\n", .{});
+            return .unresolved;
         },
         .tuple_literal => |tl| resolveTupleLiteralAsType(&tl, table),
         .parameterized_type_expr => |pt| resolveParameterizedType(&pt, table),