ir: resolveAstType unhandled-node else arm returns .unresolved, not .s64

A non-type AST node reaching type resolution is a caller bug; returning a
plausible .s64 silently fabricated an 8-byte int. Return the .unresolved
sentinel so it surfaces (and trips the sizeOf/toLLVMType panic if it ever
reaches codegen). The stderr breadcrumb stays. No test exercised this arm
(suite unchanged), so nothing was relying on the fabricated s64.

src/ir/type_bridge.zig

@@ -50,8 +50,12 @@ pub fn resolveAstType(node: ?*const Node, table: *TypeTable) TypeId {
         .struct_decl => |sd| resolveInlineStruct(&sd, table),
         .union_decl => |ud| resolveInlineUnion(&ud, table),
         else => {
-            std.debug.print("type_bridge: unhandled node type {s}\n", .{@tagName(n.data)});
-            return .s64;
+            // A non-type AST node reached type resolution — a caller bug.
+            // Returning a plausible `.s64` would silently fabricate an 8-byte
+            // int; return the `.unresolved` sentinel so it surfaces (and trips
+            // the sizeOf/toLLVMType panic if it ever reaches codegen).
+            std.debug.print("type_bridge: unhandled node type {s} in type position — returning .unresolved\n", .{@tagName(n.data)});
+            return .unresolved;
         },
     };
 }