#36647 acpmux: build on macOS again (daemon enroll off Linux)

A macOS build of acpmux fails on feat-cmux-next since https://github.com/manaflow-ai/cmux/pull/36564:

error[E0433]: cannot find `platform` in `crate`
   --> crates/acpmux/src/cli/entry.rs:214:58

The daemon enroll arm for non-Linux builds calls crate::platform::unsupported, but mod platform is #[cfg(not(unix))] (Windows only). On macOS the arm now returns its own error, daemon enroll runs only on Linux.

Red: cmux-ci dev build job 4aed088bf06e589b164bc0f8 (step tui-fetch) for https://github.com/manaflow-ai/cmux/pull/36084. That integration PR takes this commit and its dev build is the macOS check.

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Summary by cubic

Fixes macOS builds of acpmux failing after the recent daemon enroll routing change.

• On non-Linux builds, daemon enroll previously returned crate::platform::unsupported, but that module only compiles for Windows, so macOS builds hit a compile error.
• The arm now returns its own daemon enroll runs only on Linux error instead.