Emit a real framework for React-cxxstableapi under use_frameworks! (#57936)

Summary:

`react/cxxstableapi` is a header-only module — the stable API guards are pure
preprocessor headers with no compilable source. Under `use_frameworks!`,
CocoaPods emits a `PBXAggregateTarget` for a pod with no compilable sources, and
an aggregate target produces no `.framework`, so pods depending on
`React-cxxstableapi` cannot resolve the guard headers.

Add an anchor translation unit so the pod has one source and CocoaPods emits a
real framework target instead, and widen the podspec's build-from-source glob to
pick it up. The prebuilt glob stays headers-only, since prebuilt pods do not
generate frameworks. This mirrors the existing `CSSDummy.cpp` anchor for the
header-only `React-renderercss` pod.

Also register `react/cxxstableapi` as a subdirectory of the Fantom tester CMake
build so the guard headers resolve there.

Changelog: [Internal]

Differential Revision: D115859623

Author: coado

packages/react-native/ReactCommon/react/cxxstableapi/React-cxxstableapi.podspec

@@ -25,7 +25,7 @@ Pod::Spec.new do |s|
   s.author                 = "Meta Platforms, Inc. and its affiliates"
   s.platforms              = min_supported_versions
   s.source                 = source
-  s.source_files           = podspec_sources("*.h", "*.h")
+  s.source_files           = podspec_sources("*.{cpp,h}", "*.h")
   s.header_dir             = "react/cxxstableapi"
   s.pod_target_xcconfig    = { "CLANG_CXX_LANGUAGE_STANDARD" => rct_cxx_language_standard(),
                                "DEFINES_MODULE" => "YES" }

packages/react-native/ReactCommon/react/cxxstableapi/StableApiAnchor.cpp

@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+// Anchor translation unit for the `react/cxxstableapi` pod.
+//
+// The guards in this module are pure preprocessor headers, so the pod would
+// otherwise ship no compilable source. CocoaPods emits a PBXAggregateTarget for
+// a pod with no compilable sources, and an aggregate target produces no
+// `.framework` under `use_frameworks!` — dependents are then unable to resolve
+// the guard headers. Giving the pod a single source file forces CocoaPods to
+// emit a real framework target instead.
+//
+// This file intentionally declares nothing: `react/cxxstableapi` has no runtime
+// API, only preprocessor guards. Do not add code here.

private/react-native-fantom/tester/CMakeLists.txt

@@ -58,6 +58,7 @@ add_react_common_subdir(jsitooling)
 add_react_common_subdir(logger)
 add_react_common_subdir(oscompat)
 add_react_common_subdir(react/bridging)
+add_react_common_subdir(react/cxxstableapi)
 add_react_common_subdir(react/debug)
 add_react_common_subdir(react/featureflags)
 add_react_common_subdir(react/nativemodule/core)