feat: write the site for Starlight behind a flag

generate_site.py --engine starlight writes the pages, navigation and
data files of an Astro Starlight project in website/. Links resolve to
site addresses, every heading keeps its MkDocs id, and pages split by
size are measured in the MkDocs Markdown so both builds cut the same
files. The 839 pages and their 45,613 ids match the MkDocs build.

website/tools/lighthouse.py

@@ -0,0 +1,57 @@
+#!/usr/bin/env python3
+"""Lighthouse mobile (default form factor) on a list of pages, one summary line each.
+
+    python lighthouse.py <out dir> <name>=<base url> ... -- <page> ...
+
+Needs `npx lighthouse` and Chrome at /usr/bin/google-chrome.
+"""
+from __future__ import annotations
+
+import json
+import os
+import subprocess
+import sys
+from pathlib import Path
+
+CATEGORIES = ("performance", "accessibility", "best-practices", "seo")
+
+
+def main() -> int:
+    args = sys.argv[1:]
+    out = Path(args.pop(0))
+    split = args.index("--")
+    engines = dict(item.split("=", 1) for item in args[:split])
+    pages = args[split + 1:]
+    out.mkdir(parents=True, exist_ok=True)
+    env = {**os.environ, "CHROME_PATH": "/usr/bin/google-chrome"}
+    for name, base in engines.items():
+        for page in pages:
+            slug = page.strip("/").replace("/", "_") or "home"
+            report = out / f"{name}-{slug}.json"
+            # Under WSL chrome-launcher names its profile folder with a Windows path,
+            # created relative to the working directory: run it inside the output folder.
+            subprocess.run(
+                ["npx", "--prefix", str(Path(__file__).resolve().parent.parent), "lighthouse", base + page,
+                 "--quiet", "--output=json", f"--output-path={report.resolve()}",
+                 "--chrome-flags=--headless=new --no-sandbox",
+                 f"--only-categories={','.join(CATEGORIES)}"],
+                env=env, cwd=out, check=False, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+            data = json.loads(report.read_text())
+            cats, audits = data["categories"], data["audits"]
+            failed = [k for k, a in audits.items()
+                      if a.get("score") is not None and a["score"] < 1 and a.get("scoreDisplayMode") == "binary"]
+            print(json.dumps({
+                "engine": name, "page": page or "/",
+                **{c: round(cats[c]["score"] * 100) for c in CATEGORIES},
+                "lcp": audits["largest-contentful-paint"]["displayValue"],
+                "tbt": audits["total-blocking-time"]["displayValue"],
+                "cls": audits["cumulative-layout-shift"]["displayValue"],
+                "bytes": audits["total-byte-weight"]["displayValue"],
+                "dom": audits["dom-size"]["displayValue"],
+                "failed": failed,
+            }))
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())