Harden first-release validation and dependency hygiene

tools/check_format.py

@@ -0,0 +1,9 @@
+"""Check every Rust source file, including include! fragments."""
+import subprocess
+from pathlib import Path
+
+root = Path(__file__).resolve().parents[1]
+files = sorted(p for folder in ("src", "tests") for p in (root / folder).rglob("*.rs"))
+subprocess.run(["cargo", "fmt", "--all", "--check"], cwd=root, check=True)
+subprocess.run(["rustfmt", "--check", "--edition", "2024", *map(str, files)],
+               cwd=root, check=True)