muxg: build, run and gate on macOS — the TUI is the only client there

open   by a73x

On a Mac the TUI is the only client. `make release-mac` lists `muxg` in the tarball recipe, but nothing exercises the GUI there: `test/mac.sh` and `test/xos.sh` never build or run `muxg`, `docs/decisions.md` records no Darwin measurement of it, and build.zig's `native-stress` is Linux-only by its own description. The GUI needs SDL3, freetype, fontconfig and HarfBuzz plus GL headers from the host, and on a Mac those come from Homebrew against the `deps/mac-sdk` shadow SDK, which zig 0.15.2 may or may not link.

Wanted: `make native` and `make native-core-test` green on MAC_BUILDER; `muxg` in the macOS tarball verified to launch a window and attach to a local `mux d`; a `mac.sh` leg that does so under the same scrubbed-box rule as the TUI journeys.

This is the first gate on retiring the terminal wall: until it lands, deleting the wall leaves macOS with a single-session terminal client and no way to see two sessions at once.