Add Flutter channel shader bundle smoke shards

[bdero]

• add a fixture Flutter app that uses a real hook/build.dart and shader bundle manifest
• add tool/verify_shader_bundle_hook.sh to build the fixture, verify bundle output, verify unchanged builds skip build_hooks, and verify shader edits rerun build_hooks
• add dedicated GitHub Actions PR shards for Flutter master and Flutter stable
• keep the master shard checking transitive #include invalidation via depfile support, while the stable shard checks the backwards-compatible direct shader dependency path