feat: add logical import support with wren_modules directory resolution and path type classification

Introduce a path type classification system (absolute, relative, simple) to distinguish logical imports from relative ones. Implement automatic discovery of a "wren_modules" directory by walking up from the root directory. Refactor the CLI to return interpret results and exit codes instead of exiting directly. Extract cross-platform stat macros into a shared header. Update test API calls to use explicit relative paths.

src/vm/wren_vm.c

@@ -723,7 +723,7 @@ static Value resolveModule(WrenVM* vm, Value name)
   return name;
 }
 
-Value wrenImportModule(WrenVM* vm, Value name)
+static Value importModule(WrenVM* vm, Value name)
 {
   name = resolveModule(vm, name);
   
@@ -1308,7 +1308,7 @@ static WrenInterpretResult runInterpreter(WrenVM* vm, register ObjFiber* fiber)
     {
       Value name = fn->constants.data[READ_SHORT()];
 
-      Value result = wrenImportModule(vm, name);
+      Value result = importModule(vm, name);
       if (!IS_NULL(fiber->error)) RUNTIME_ERROR();
 
       // Make a slot on the stack for the module's closure to place the return