fix: declare pyproj as a direct dependency

pyproject.toml

@@ -24,6 +24,7 @@ dependencies = [
   "matplotlib>=3.9",
   "numpy>=2.0",
   "pandas>=2.2",
+  "pyproj>=3.6",
   "pyyaml>=6.0",
   "rebound>=4.6,<5.0",
   "reboundx>=4.6,<5.0",

tests/test_declared_dependencies.py

@@ -0,0 +1,35 @@
+"""Guard that third-party imports used by the package are declared dependencies.
+
+``chained_eclipse.search`` and ``chained_eclipse.sensitivity`` both do
+``from pyproj import Geod``. Before this test, ``pyproj`` was installed only as a
+transitive dependency of ``cartopy``; if cartopy ever dropped or vendored it,
+these imports would fail at runtime with no declared-dependency safety net.
+"""
+
+from __future__ import annotations
+
+import importlib
+import tomllib
+from pathlib import Path
+
+import pytest
+
+PYPROJECT = Path(__file__).resolve().parents[1] / "pyproject.toml"
+
+
+@pytest.mark.parametrize(
+    "module_name",
+    ["chained_eclipse.search", "chained_eclipse.sensitivity"],
+)
+def test_pyproj_dependent_modules_import(module_name: str) -> None:
+    assert importlib.import_module(module_name) is not None
+
+
+@pytest.mark.skipif(not PYPROJECT.exists(), reason="running from an installed wheel")
+def test_pyproj_is_a_declared_runtime_dependency() -> None:
+    metadata = tomllib.loads(PYPROJECT.read_text(encoding="utf-8"))
+    declared = metadata["project"]["dependencies"]
+    assert any(spec.split(">")[0].split("=")[0].strip() == "pyproj" for spec in declared), (
+        "pyproj is imported by chained_eclipse.search and chained_eclipse.sensitivity "
+        "and must be declared in [project] dependencies, not relied on transitively."
+    )