probe: drive xlings directly to see whether the 127 is its own

mcpp returns the xlings child's exit code on this path and swallows its stderr,
and the pinned xlings differs sharply across the versions under comparison
(0.0.102 -> 0.4.51, 0.0.104+ -> 0.4.68/0.4.69) — which makes xlings, not mcpp,
the thing this run needs to rule in or out.

Author: Sunrisepeak

.github/workflows/validate.yml

@@ -369,6 +369,18 @@ jobs:
           "$MCPP" test -p tests/examples/ffmpeg || rc=$?
           echo "=== ffmpeg member exit status: $rc ==="
           echo "=== free space after ==="; df -h . || true
+          # mcpp returns the xlings child's exit code, so 127 may well be
+          # xlings'. Drive xlings directly on the same package and show what it
+          # says — mcpp swallows the child's stderr on this path.
+          echo "=== locating xlings ==="
+          XL=$(find . "$HOME" -name 'xlings.exe' -path '*registry/bin*' 2>/dev/null | head -1)
+          echo "xlings: $XL"
+          if [ -n "$XL" ]; then
+            "$XL" --version || true
+            xrc=0
+            "$XL" install "compat:ffmpeg@8.1.2" -y || xrc=$?
+            echo "=== xlings direct exit status: $xrc ==="
+          fi
           exit $rc
           # No `timeout` wrapper: absent on macOS runners; job-level timeout-minutes bounds it.
           if [ "$MEMBERS" = "__ALL__" ]; then