Isolate Rust implementation under top-level rust/ directory

.gitignore

@@ -8,6 +8,7 @@ __pycache__/
 *.egg-info/
 build/
 dist/
+target/
 .pytest_cache/
 .ruff_cache/
 rulezet-validation.toml

CONTRIBUTING.md

@@ -30,8 +30,13 @@ src/rulezet_validation/
   sync.py       orchestration
   cli.py        argparse wiring only, no logic
   baseline/     manifest + the bundled probes
+rust/            all Rust crates and Rust implementation
 ```
 
+Rust source and Cargo manifests belong under the top-level `rust/` directory.
+The repository layout test enforces this boundary so Rust tooling does not become
+interspersed with the Python package.
+
 ## Things that will get a patch rejected
 
 **Anything other than a baseline hit moving a rule into quarantine.** The

rust/README.md

@@ -0,0 +1,8 @@
+# Rust implementation
+
+All Rust crates, Cargo manifests, and Rust source code for this project live in
+this directory. Keeping the Rust implementation here gives Cargo an isolated
+workspace while leaving the Python package under `src/rulezet_validation/`.
+
+New Rust crates should be created as children of this directory. Do not place
+`Cargo.toml`, `Cargo.lock`, or `*.rs` files elsewhere in the repository.

tests/test_repository_layout.py

@@ -0,0 +1,21 @@
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[1]
+RUST_ROOT = ROOT / "rust"
+RUST_FILES = {"Cargo.toml", "Cargo.lock"}
+
+
+def test_rust_implementation_is_contained_in_rust_directory():
+    misplaced = []
+
+    for path in ROOT.rglob("*"):
+        if any(part in {".git", ".venv", "target"} for part in path.parts):
+            continue
+        if path.is_file() and (path.suffix == ".rs" or path.name in RUST_FILES):
+            if not path.is_relative_to(RUST_ROOT):
+                misplaced.append(path.relative_to(ROOT).as_posix())
+
+    assert misplaced == [], (
+        "Rust source and Cargo files must live under rust/: "
+        + ", ".join(misplaced)
+    )