fix(ci): green up the test job on a fresh clone

Two bugs in test_phase0_gate.py surfaced in CI but never locally
because my dev tree predates the issues:

  1. test_directory_structure asserts that
     ``defaultplusplus/src/defaultplusplus/pretrained/weights/`` is
     a directory. It was empty so git never tracked it. Add a
     .gitkeep so the directory survives a fresh clone.

  2. test_pyproject_toml does ``import tomllib`` unconditionally.
     ``tomllib`` is stdlib only on Python 3.11+. Fall back to
     ``tomli`` on 3.10 (the upstream of tomllib with the identical
     API). Added ``tomli>=2.0; python_version < '3.11'`` to the
     [dev] extra so it installs only where needed.

Local suite: 150 passed, no regressions. Should clear the matrix on
3.10 / 3.11 / 3.12.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: SigmaJahan

defaultplusplus/pyproject.toml

@@ -92,6 +92,9 @@ dev = [
     "ruff>=0.4",
     "build>=1.0",
     "twine>=5.0",
+    # Python 3.11 ships ``tomllib`` in the standard library; on 3.10 we
+    # fall back to ``tomli``, which is its identical-API upstream.
+    "tomli>=2.0; python_version < '3.11'",
 ]
 # Convenience extra for installing the full research stack.
 all = [

defaultplusplus/src/defaultplusplus/pretrained/weights/.gitkeep

@@ -0,0 +1,3 @@
+# This file exists so the empty weights/ directory is tracked by git.
+# It is the destination for pretrained diagnostic-model weights, which
+# are downloaded on first use rather than shipped in the wheel.

defaultplusplus/tests/test_phase0_gate.py

@@ -52,7 +52,13 @@ def test_pyproject_toml():
     Version is read dynamically from ``defaultplusplus._version`` so
     it is not a literal field in ``[project]``.
     """
-    import tomllib
+    # tomllib only landed in Python 3.11; on 3.10 we fall back to the
+    # tomli package, which is the upstream of tomllib and has the
+    # identical API.
+    try:
+        import tomllib  # type: ignore[import-not-found]
+    except ImportError:
+        import tomli as tomllib  # type: ignore[no-redef]
 
     toml_path = ROOT / "pyproject.toml"
     assert toml_path.exists()