window-union: a valid placement with no host selections divides by zero #656

Problem

A valid placement that leaves no selected expert on the host makes tools/ref/window-union.py terminate with ZeroDivisionError while printing its prediction column.

Observed on unchanged main 69b66367 and on my #637 placement-input branch. This is separate from the negative/duplicate input and exception-identity fixes; those do not change the report arithmetic.

Reproduction (Python 3, no model or GPU)

python3 - <<'PY'
import importlib.util, pathlib, subprocess, sys, tempfile
spec = importlib.util.spec_from_file_location("wu", "tools/ref/window-union.py")
wu = importlib.util.module_from_spec(spec)
spec.loader.exec_module(wu)
with tempfile.TemporaryDirectory() as root:
    trace = wu._fake_set(root, "set", {0: [(3, 7)] * 8, 1: [(3, 7)] * 8}, 16, 2)
    result = subprocess.run([sys.executable, "tools/ref/window-union.py", trace,
                             "--n-l", "spread:32@0-1", "--k", "2"])
    raise SystemExit(result.returncode)
PY

The placement admits 16 experts on each of two layers, all within the valid range. The command prints host slots per row 0.0 of 4, then exits 1 with ZeroDivisionError: float division by zero.

Cause / expected behavior

report, at the pinned main unconditionally formats pu / ps; ps is zero when there are no host selections. The adjacent summary already treats undefined ratios as NaN. The report should represent the no-host-work case explicitly, rather than crash or pretend a meaningful ratio was measured. A no-host-selection case can also arise without every expert being card-resident.

The separate #637 work remains limited to its three claimed CPU-tool items. No performance or NVIDIA execution claim.