Stubbed Swift typecheck harnesses hide real compile errors

When typechecking a single Swift app file without Xcode (swiftc -typecheck with stub files), stubbing out the types a file calls means those calls are never checked: 'cannot find X' errors look like harness noise while a real error (e.g. passing a non-escaping closure where @escaping is required) slips to CI. Compile against the real dependency source files, not stubs, for the call sites you changed. Also pass -target arm64-apple-macos<deployment target> matching the app, or availability errors such as NSMenuItem.subtitle (macOS 14.4+) only appear in CI.