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>

docs/TESTING.md

@@ -60,6 +60,7 @@ diff -r /tmp/before /tmp/after
 
 ## Browser tests
 
+- They need the `.venv` from README.md → Develop; plain `python3` switches to it on its own.
 - `test/play.py` (about 2.5 minutes) builds a world through the title screen and plays it with real
   input. It checks the tools, building, copy/cut/paste, undo, the build menu, filters, pipes,
   the notebook and market, saving and continuing, exporting and importing a world file, the