Honor explicit BONSAI_VARIANT during M5 metallib swap gating in setup.sh

[Copilot]

On M5 systems with the NAX miscompile, setup.sh could detect the issue but still skip the ternary metallib swap when both model variants existed locally. The script unconditionally re-inferred binary from models/ and overrode an explicit BONSAI_VARIANT=ternary.

• Variant resolution behavior

  • Keep _variant="${BONSAI_VARIANT:-ternary}" as the primary source of truth.
  • Only run binary directory auto-detection when BONSAI_VARIANT is unset.

• Impact on swap path

  • Preserves current auto-detect behavior for default runs.
  • Allows explicit BONSAI_VARIANT=ternary to reach the ternary metallib swap branch instead of being silently forced to binary.

_variant="${BONSAI_VARIANT:-ternary}"
if [ -z "${BONSAI_VARIANT:-}" ]; then
  for _bin_model in "$SCRIPT_DIR"/models/bonsai-image-4B-binary-*; do
    [ -d "$_bin_model" ] && _variant="binary" && break
  done
fi

[Copilot]

Pull request overview

This PR fixes setup.sh variant resolution so an explicitly set BONSAI_VARIANT is not overridden by local models/ auto-detection, ensuring the intended metallib swap gating behavior on affected M5 systems.

Changes:

• Preserve BONSAI_VARIANT as the primary source of truth for _variant.
• Run binary-model auto-detection only when BONSAI_VARIANT is unset.

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[bri-prism]

The fix is correct and nicely scoped. Gating the binary auto-detection behind [ -z "${BONSAI_VARIANT:-}" ] is the right call, since an explicit BONSAI_VARIANT=ternary now reaches the metallib swap branch instead of being clobbered when a binary model happens to be sitting in models/.

I walked through the three cases and they all hold:

• Unset: defaults to ternary, then auto-detect can promote it to binary, same as before.
• ternary: stays ternary, detection skipped. That is the case we wanted to fix.
• binary: stays binary, detection skipped.

One minor note, not a blocker: when no binary model is present the glob still falls through cleanly because the directory test rejects the literal pattern, so that path is unchanged.

Looks good to me.