fix(cocoapods): define podspec_dir in RNCoreFacades.generate

The FACADE_REEXPOSED_HEADERS loop passed an undefined `podspec_dir` to
copy_reexposed_headers, crashing `pod install` at `use_react_native!`
("undefined local variable or method 'podspec_dir'"). Derive it from the
real podspec path.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: chrfalch

packages/react-native/scripts/cocoapods/rncore_facades.rb

@@ -114,6 +114,7 @@ def self.generate(react_native_path, install_root, version, ios_version)
         FileUtils.mkdir_p(abs_base)
         FACADE_PODS.each do |name, podspec_rel_path|
             podspec_path = File.join(react_native_path.to_s, podspec_rel_path)
+            podspec_dir = File.dirname(podspec_path)
             real = load_real_spec(podspec_path, name)
             dir = File.join(abs_base, name)
             FileUtils.mkdir_p(dir)