Fix size-matrix CI: skip WAT-dependent differential tests when WAT disabled

The two loadFromWat-based differential tests (fibonacci, loop) lacked
the enable_wat guard that all other WAT tests in types.zig already have.
This caused size-matrix job failure with -Dwat=false.

Author: chaploud

src/vm.zig

@@ -9954,6 +9954,7 @@ test "Differential — i32.add interpreter vs JIT" {
 }
 
 test "Differential — recursive fibonacci interpreter vs JIT" {
+    if (!build_options.enable_wat) return error.SkipZigTest;
     const types = @import("types.zig");
     const WasmModule = types.WasmModule;
     const mod = try WasmModule.loadFromWat(testing.allocator,
@@ -9972,6 +9973,7 @@ test "Differential — recursive fibonacci interpreter vs JIT" {
 }
 
 test "Differential — loop with accumulator interpreter vs JIT" {
+    if (!build_options.enable_wat) return error.SkipZigTest;
     const types = @import("types.zig");
     const WasmModule = types.WasmModule;
     const mod = try WasmModule.loadFromWat(testing.allocator,