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>

test/t_smooth.py

@@ -1,5 +1,7 @@
 """The smooth look: the WebGL pass compiles, draws, falls back cleanly, and the thermal lens still works."""
 import sys, pathlib
+sys.path.insert(0, str(pathlib.Path(__file__).resolve().parent.parent / 'tools'))
+from use_venv import ensure_playwright; ensure_playwright()
 from playwright.sync_api import sync_playwright
 root = pathlib.Path(__file__).resolve().parent.parent
 out = pathlib.Path(sys.argv[1] if len(sys.argv) > 1 else root / 'shots' / 'smooth'); out.mkdir(parents=True, exist_ok=True)