Run browser tests locally: Playwright 1.64.1 in .venv, scripts switch to it on their own

The browser tests and screenshot tools now work with plain python3 on a machine whose
system Python refuses pip installs: tools/use_venv.py re-runs them with the repo's .venv.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/use_venv.py

@@ -0,0 +1,18 @@
+"""Let browser scripts run with plain python3 by switching to the repo's .venv."""
+import os, pathlib, sys
+
+ROOT = pathlib.Path(__file__).resolve().parent.parent
+
+def ensure_playwright():
+    try:
+        import playwright
+        return
+    except ImportError:
+        pass
+    venv = ROOT / '.venv'
+    py = venv / 'bin' / 'python'
+    if py.exists() and pathlib.Path(sys.prefix).resolve() != venv.resolve():
+        os.execv(str(py), [str(py), *sys.argv])
+    sys.exit('Playwright is not installed; see Develop in README.md (uv venv .venv, '
+             'then uv pip install --python .venv/bin/python -r requirements.txt, '
+             'then .venv/bin/python -m playwright install chromium).')