Recognise the engine wheel's real layout

src/simantic/install.py

@@ -302,7 +302,22 @@ def rust_engine_root() -> Path:
 
 
 def is_rust_engine(d: Path) -> bool:
-    return d.is_dir() and any(p.name.startswith("simantic_rust.") for p in d.iterdir())
+    """True when `d` holds an importable `simantic_rust`.
+
+    maturin ships the extension as a package: `simantic_rust/__init__.py`
+    beside `simantic_rust/simantic_rust.abi3.so`. A bare `simantic_rust.<ext>`
+    laid out flat is equally importable. Accept either, because the only thing
+    that matters here is whether putting `d` on `sys.path` makes the module
+    import; matching one layout silently re-downloaded the engine on every
+    process, since the managed copy was never recognised.
+    """
+    if not d.is_dir():
+        return False
+    return any(
+        p.name.startswith("simantic_rust.")
+        or (p.name == "simantic_rust" and (p / "__init__.py").exists())
+        for p in d.iterdir()
+    )
 
 
 def installed_rust_engine() -> Path | None:

tests/test_install.py

@@ -434,3 +434,32 @@ def capture(request, **k):
     with pytest.raises(install.InstallError):
         install.fetch_rust_manifest()
     assert seen[0].endswith("/pyrite/latest.json")
+
+
+# -- engine layouts ----------------------------------------------------------
+#
+# maturin ships the Rust engine as a package, not a bare .so. Recognising only
+# the flat shape looked harmless (the import still worked once the directory was
+# on sys.path) but meant the managed copy was never found, so every process
+# re-downloaded the wheel.
+
+
+def test_a_maturin_package_layout_is_recognised(tmp_path):
+    """What `maturin build` actually produces, verified against a real wheel."""
+    pkg = tmp_path / "simantic_rust"
+    pkg.mkdir()
+    (pkg / "__init__.py").write_text("from .simantic_rust import *\n")
+    (pkg / "simantic_rust.abi3.so").write_bytes(b"\x7fELF")
+    (tmp_path / "simantic_rust-0.3.0.dist-info").mkdir()
+    assert install.is_rust_engine(tmp_path)
+
+
+def test_a_flat_extension_is_recognised(tmp_path):
+    (tmp_path / "simantic_rust.abi3.so").write_bytes(b"\x7fELF")
+    assert install.is_rust_engine(tmp_path)
+
+
+def test_a_directory_without_the_module_is_not(tmp_path):
+    (tmp_path / "simantic_rust-0.3.0.dist-info").mkdir()
+    (tmp_path / "simantic_rust").mkdir()          # no __init__.py: not importable
+    assert not install.is_rust_engine(tmp_path)