Gerüst für den Rust-Kern anlegen

Workspace, leeres Crate grovewright-core und ein Wächter-Test, der den
Quelltext des Kerns auf Verstöße gegen die Gleichschritt-Regeln prüft.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

core/tests/forbidden_apis.rs

@@ -0,0 +1,78 @@
+//! Guards the lockstep rules: the core must not use anything whose result
+//! can differ between machines or between runs.
+
+use std::fs;
+use std::path::{Path, PathBuf};
+
+/// Text that must not appear anywhere in the source of the core, and why.
+const FORBIDDEN: &[(&str, &str)] = &[
+    ("f32", "floating point results can differ between machines"),
+    ("f64", "floating point results can differ between machines"),
+    (
+        "HashMap",
+        "iteration order differs between runs; use BTreeMap or Vec",
+    ),
+    (
+        "HashSet",
+        "iteration order differs between runs; use BTreeSet or Vec",
+    ),
+    ("std::time", "the core must not read the clock"),
+    ("Instant", "the core must not read the clock"),
+    ("SystemTime", "the core must not read the clock"),
+    ("std::thread", "the core must stay single-threaded"),
+    ("std::env", "the core must not depend on the environment"),
+    (
+        "std::fs",
+        "the core must not touch files; the shell does that",
+    ),
+    (
+        "std::net",
+        "the core must not open connections; the shell does that",
+    ),
+    ("unsafe", "the core forbids unsafe code"),
+];
+
+fn rust_files(dir: &Path, out: &mut Vec<PathBuf>) {
+    for entry in fs::read_dir(dir).expect("source directory is readable") {
+        let path = entry.expect("directory entry is readable").path();
+        if path.is_dir() {
+            rust_files(&path, out);
+        } else if path.extension().is_some_and(|ext| ext == "rs") {
+            out.push(path);
+        }
+    }
+}
+
+#[test]
+fn core_source_uses_no_forbidden_apis() {
+    let src = Path::new(env!("CARGO_MANIFEST_DIR")).join("src");
+    let mut files = Vec::new();
+    rust_files(&src, &mut files);
+    files.sort();
+    assert!(
+        !files.is_empty(),
+        "no source files found in {}",
+        src.display()
+    );
+
+    let mut problems = Vec::new();
+    for file in &files {
+        let text = fs::read_to_string(file).expect("source file is readable");
+        for (number, line) in text.lines().enumerate() {
+            for (needle, reason) in FORBIDDEN {
+                if line.contains(needle) {
+                    problems.push(format!(
+                        "{}:{}: `{needle}`: {reason}",
+                        file.display(),
+                        number + 1
+                    ));
+                }
+            }
+        }
+    }
+    assert!(
+        problems.is_empty(),
+        "forbidden APIs in the core:\n{}",
+        problems.join("\n")
+    );
+}