Add macOS support to unified/swift-syntax-rs build

[Copilot]

The unified/swift-syntax-rs crate's Bazel build and local cargo path were Linux-only. This adds macOS (xcode) support to both while keeping Ubuntu working.

Changes

• MODULE.bazel — Registers the xcode toolchain alongside ubuntu24.04:

  use_repo(swift, "swift_toolchain", "swift_toolchain_ubuntu24.04", "swift_toolchain_xcode")
  register_toolchains(
      "@swift_toolchain//:swift_toolchain_exec_ubuntu24.04",
      "@swift_toolchain//:swift_toolchain_exec_xcode",
  )

  Comment updated to note the macOS .pkg can only be fetched/extracted on a macOS host.

• unified/swift-syntax-rs/BUILD.bazel — Replaces hardcoded @swift_toolchain_ubuntu24.04//:files on both rust_binary and rust_test with a platform select():

  data = select({
      "@platforms//os:macos": ["@swift_toolchain_xcode//:files"],
      "//conditions:default": ["@swift_toolchain_ubuntu24.04//:files"],
  }),

  The :files target is confirmed valid for the xcode repo — rules_swift 4.0.0-rc4 uses the same BUILD.bazel.tpl for all platforms.

• unified/swift-syntax-rs/build.rs — swift_runtime_dir() now selects the OS-appropriate subdirectory (macosx on macOS, linux otherwise) instead of always using linux.

• unified/swift-syntax-rs/README.md — Updates "Building with Bazel (CI)" to reflect both registered platforms and the macOS .pkg host constraint.