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/perf.py

@@ -4,6 +4,8 @@ light and glow), the pixel upload and blit, and one simulation tick, in ms. Rebu
   python3 tools/perf.py
 """
 import sys, pathlib
+sys.path.insert(0, str(pathlib.Path(__file__).resolve().parent))
+from use_venv import ensure_playwright; ensure_playwright()
 from playwright.sync_api import sync_playwright
 root = pathlib.Path(__file__).resolve().parent.parent
 url = (root / 'dist' / 'index.html').as_uri()