CI: syntax-check every plugin's native watchOS sources with Xcode 26 and 27 #17

CI never compiles packages/*/watchos/Classes/*.m. In #12, a symbol declared only in the watchOS 27 SDK (CLActivityTypeMaritime) would have broken every Xcode 26 build, and CI would have stayed green.

Proposal: a macOS job that runs xcrun --sdk watchos clang -fsyntax-only (with the CLI's flags) over each package's .m files, for device and simulator, on Xcode 26.x and 27.x.