fix(adr-117/p5): pin Python 3.12 + isolated venv for tombstone smoke-test

First v1.99.0-pip run (26366491748) failed: the runner's system `python`
fell back to `--user` install, then `python -c "import wifi_densepose"`
resolved to something other than the freshly-installed user-site wheel
and returned cleanly instead of raising the tombstone ImportError.

Fixes:
- `actions/setup-python@v5` with explicit 3.12 — owns its own site-
  packages so pip won't fall back to --user.
- New "Inspect wheel contents" step prints the wheel manifest +
  the verbatim __init__.py inside it. If a future regression ships
  an empty __init__.py from a setuptools src-layout edge case,
  the failure is debuggable from the run log alone.
- Smoke test now runs in a fresh /tmp/smoke-venv so there's zero
  ambiguity about which wifi_densepose gets imported. Also uses
  importlib.util.find_spec to print the resolved origin path
  before the import attempt — so even if both checks pass, we
  see exactly which file we exercised.

No code changes to the tombstone source itself.

Co-Authored-By: claude-flow <ruv@ruv.net>

.github/workflows/pip-release.yml

@@ -147,24 +147,53 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
       - name: Install build backend
-        run: pip install build>=1.2
+        run: python -m pip install --upgrade pip build>=1.2
       - name: Build sdist + wheel
         working-directory: python/tombstone
         run: python -m build --outdir ../../tombstone-dist
-      # Smoke-test: the wheel MUST raise ImportError on import.
-      - name: Smoke-test tombstone
+      # Inspect what was actually built — the previous v1.99.0-pip run
+      # showed an `import wifi_densepose` that returned cleanly instead
+      # of raising, even though build logs said `adding 'wifi_densepose/__init__.py'`.
+      # Print the wheel manifest + the __init__.py content so any
+      # future regression is debuggable from the run log alone.
+      - name: Inspect wheel contents
         run: |
-          python -m pip install tombstone-dist/wifi_densepose-1.99.0-py3-none-any.whl
+          set -e
+          WHL=tombstone-dist/wifi_densepose-1.99.0-py3-none-any.whl
+          echo "--- wheel listing ---"
+          python -m zipfile -l "$WHL"
+          echo "--- wifi_densepose/__init__.py inside the wheel ---"
+          python -m zipfile -e "$WHL" /tmp/tomb-inspect
+          cat /tmp/tomb-inspect/wifi_densepose/__init__.py
+          echo "--- size in bytes ---"
+          wc -c /tmp/tomb-inspect/wifi_densepose/__init__.py
+      # Smoke-test in an ISOLATED venv. The previous run's failure
+      # mode was that the ubuntu-latest runner's system `python` had
+      # site-packages picking up something other than the user-installed
+      # wheel, so the import resolved to a different module. A clean
+      # venv removes any ambiguity about which wifi_densepose is loaded.
+      - name: Smoke-test tombstone in isolated venv
+        run: |
+          set -e
+          python -m venv /tmp/smoke-venv
+          /tmp/smoke-venv/bin/python -m pip install --upgrade pip
+          /tmp/smoke-venv/bin/python -m pip install tombstone-dist/wifi_densepose-1.99.0-py3-none-any.whl
+          # Show where wifi_densepose actually resolved + that the file
+          # the venv would import is the tombstone, NOT something else
+          # on the path.
+          /tmp/smoke-venv/bin/python -c "import importlib.util as u; s = u.find_spec('wifi_densepose'); print('Resolved to:', s.origin); print('--- file content ---'); print(open(s.origin).read())"
           set +e
-          python -c "import wifi_densepose" 2> import-output.txt
+          /tmp/smoke-venv/bin/python -c "import wifi_densepose" 2> import-output.txt
           rc=$?
           set -e
           if [ "$rc" -eq 0 ]; then
             echo "ERROR: tombstone import succeeded — should have raised ImportError"
             exit 1
           fi
-          # Must include the migration URL so users can find their way home.
           if ! grep -q "github.com/ruvnet/RuView" import-output.txt; then
             echo "ERROR: tombstone ImportError missing migration URL"
             cat import-output.txt