Add macOS support to unified/swift-syntax-rs (Bazel + cargo paths)

[Copilot]

unified/swift-syntax-rs was Linux-only: the Bazel build hardcoded @swift_toolchain_ubuntu24.04//:files and build.rs always linked against the linux Swift runtime subdir. This adds full macOS support (Apple Silicon and Intel) for both the Bazel and local cargo paths.

Changes

• MODULE.bazel — Adds apple_support dep; registers xcode_configure and apple_cc_configure extensions so the xcode_transition can materialise @local_config_xcode and @local_config_apple_cc_toolchains.

• unified/swift-syntax-rs/xcode_transition.bzl (new) — Starlark incoming-edge transition applied to the swift_library target only. On macOS it injects --xcode_version_config=@local_config_xcode//:host_xcodes and prepends @local_config_apple_cc_toolchains//:all to --extra_toolchains (needed because rules_swift's xcode_swift_toolchain reads cc_toolchain.target_gnu_system_name, which is "local" under Bazel's default local_config_cc). Is a no-op on Linux, so downstream Rust targets and the rest of the repo are unaffected.

• unified/swift-syntax-rs/BUILD.bazel — Replaces swift_library with xcode_transition_swift_library. Replaces the hardcoded data = ["@swift_toolchain_ubuntu24.04//:files"] on rust_binary and rust_test with:

  data = select({
      "@platforms//os:macos": [],          # OS provides /usr/lib/swift at runtime
      "@platforms//os:linux": ["@swift_toolchain_ubuntu24.04//:files"],
  }),

  All targets gain target_compatible_with so Windows (and other unsupported platforms) are skipped cleanly by bazel build //....

• unified/swift-syntax-rs/build.rs — swift_runtime_dir() now selects macosx vs linux at compile time via cfg!(target_os = "macos").

• unified/swift-syntax-rs/swift/Package.swift — Adds explicit platforms: [.macOS(.v10_15)] (swift-syntax 602 requires it; without it SPM defaults to 10.13 which the package rejects).