ci(temp): authoritative opencv-fetch diagnostics (whole-runner + direct cmake install)

Prior diag looked only under $HOME/.mcpp, but the vendored xlings installs into
its own home ($root/registry). Search the whole runner for xim-x-cmake + opencv
build log, and directly run the vendored xlings 'install cmake@4.0.2' with visible
output to capture the real xim:cmake failure (its install error is otherwise
suppressed). Remove before merge.

Author: Sunrisepeak

.github/workflows/validate.yml

@@ -199,24 +199,25 @@ jobs:
       # install() subprocess + log output, so a failed source-build (e.g. opencv's
       # CMake hook) is invisible. On failure, surface the on-disk build log + the
       # xim:cmake launcher/loader state directly from the runner.
-      - name: install() build-log on failure
+      - name: install() diagnostics on failure
         if: failure() && runner.os == 'Linux'
         shell: bash
         run: |
-          echo "=== opencv build log(s) ==="
-          find "$HOME/.mcpp" -name 'mcpp_opencv_build.log' 2>/dev/null | while read -r f; do
-            echo "--- $f ---"; cat "$f"; echo "--- end ---"
+          echo "=== env ==="
+          echo "MCPP=$MCPP"; echo "MCPP_VENDORED_XLINGS=$MCPP_VENDORED_XLINGS"; echo "HOME=$HOME"
+          echo "=== whole-runner search: xim-x-cmake / opencv build log / cmake bins ==="
+          find / -type d -name 'xim-x-cmake' 2>/dev/null | head || true
+          find / -name 'mcpp_opencv_build.log' 2>/dev/null | while read -r f; do echo "--- $f ---"; cat "$f"; echo "--- end ---"; done || true
+          find / -type f -path '*xim-x-cmake*/bin/cmake' 2>/dev/null | while read -r c; do
+            echo "cmake bin: $c"; readelf -l "$c" 2>/dev/null | grep -i interpreter || true
+            "$c" --version 2>&1 | head -2 || echo "exec FAILED"
           done || true
-          echo "=== opencv store top-level ==="
-          find "$HOME/.mcpp" -path '*compat.opencv*4.13.0' -maxdepth 9 2>/dev/null | head -1 | while read -r d; do ls -la "$d" 2>/dev/null; ls -la "$d/include/opencv4/opencv2" 2>/dev/null | head; done || true
-          echo "=== xim:cmake binary + launcher on PATH ==="
-          find "$HOME/.mcpp" -type f -name 'cmake' 2>/dev/null | head || true
-          echo "PATH=$PATH"; command -v cmake || echo "(no bare cmake on this PATH)"
-          echo "=== xim:cmake ELF interpreter + can it exec? ==="
-          cm=$(find "$HOME/.mcpp" -type f -path '*xim-x-cmake*/bin/cmake' 2>/dev/null | head -1)
-          if [ -n "$cm" ]; then readelf -l "$cm" 2>/dev/null | grep -i interpreter || true; "$cm" --version 2>&1 | head -3 || echo "exec FAILED rc=$?"; fi
-          echo "=== xim:glibc present? ==="
-          find "$HOME/.mcpp" -path '*xim-x-glibc*ld-linux*' 2>/dev/null | head || echo "(no xim glibc loader)"
+          echo "=== directly install cmake@4.0.2 via the vendored xlings (visible output) ==="
+          if [ -n "$MCPP_VENDORED_XLINGS" ] && [ -x "$MCPP_VENDORED_XLINGS" ]; then
+            MCPP_INDEX_MIRROR=GLOBAL "$MCPP_VENDORED_XLINGS" install cmake@4.0.2 -y 2>&1 | tail -40 || echo "xlings install cmake exit=$?"
+          fi
+          echo "=== opencv store (whole runner) ==="
+          find / -path '*compat.opencv*4.13.0/include/opencv4/opencv2' -type d 2>/dev/null | head || echo "(no opencv headers materialised anywhere)"
 
   # ── Exception: the public `imgui` C++23-module package (namespace "") ──
   # Not yet a workspace member: its package has an empty namespace (the builtin