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/playthrough.py

@@ -1,5 +1,7 @@
 """Headless browser run: load the page, start a world, play a little, take screenshots."""
 import sys, time, 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