Replace Python numerical kernels with native C execution

tests/test_native_only_backend.py

@@ -0,0 +1,39 @@
+"""The retired numerical backend must not become a hidden runtime dependency."""
+import os
+from pathlib import Path
+import subprocess
+import sys
+import unittest
+from unittest.mock import patch
+
+from app.simulation.backends import numeric_engine_name
+
+
+class NativeOnlyBackendTests(unittest.TestCase):
+    def test_python_backend_is_rejected_explicitly(self):
+        with patch.dict(os.environ, {'SIMULATION_NUMERIC_ENGINE': 'python'}):
+            with self.assertRaisesRegex(ValueError, 'removed'):
+                numeric_engine_name()
+        self.assertEqual(numeric_engine_name('native-c'), 'native')
+
+    def test_api_and_compiler_import_without_numpy_or_scipy(self):
+        code = '''
+import importlib.abc
+import sys
+from pathlib import Path
+class NoNumericalPython(importlib.abc.MetaPathFinder):
+    def find_spec(self, fullname, path=None, target=None):
+        if fullname.split('.')[0] in ('numpy', 'scipy'):
+            raise AssertionError('Retired numerical dependency imported: '+fullname)
+sys.meta_path.insert(0, NoNumericalPython())
+from app.main import compile_system_xml_network
+from app.simulation.native_codegen.compiler import compile_native_program
+from app.system_xml import validate_system_xml_document
+doc = validate_system_xml_document(Path('tests/data/native-skill-test.xml').read_bytes()).document
+program = compile_native_program(compile_system_xml_network(doc))
+assert len(program.state_keys) == 12
+assert len(program.variables) == 175
+assert not any(name.startswith('app.simulation.solvers.') for name in sys.modules)
+'''
+        subprocess.run([sys.executable, '-c', code], cwd=Path(__file__).resolve().parents[1],
+                       capture_output=True, text=True, check=True, timeout=30)