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>

README.md

@@ -34,10 +34,16 @@ The controls are in the in-game notebook (N) under Controls. The basics:
 You need Node (tested with 22), Python 3, and Playwright for the browser tests and screenshots:
 
 ```sh
-pip install -r requirements.txt
-python3 -m playwright install chromium
+uv venv .venv
+uv pip install --python .venv/bin/python -r requirements.txt
+.venv/bin/python -m playwright install chromium
 ```
 
+If uv is not installed, `python3 -m venv .venv` and
+`.venv/bin/pip install -r requirements.txt` also work; then install Chromium as above.
+After that, plain `python3` works for every command below; the browser scripts switch to
+`.venv` on their own.
+
 ```sh
 python3 build.py                     # build dist/index.html
 python3 test/run_all.py              # headless test suites