chore(backend-testing-01): create test directory structure

- tests/ root with __init__.py
- tests/unit/ for mocked unit tests (CI)
- tests/integration/ for hardware tests (local only)
- tests/fixtures/ for test data files

tests/__init__.py

@@ -0,0 +1 @@
+# Dune Weaver Backend Tests

tests/integration/__init__.py

@@ -0,0 +1 @@
+# Integration tests - hardware tests, local only

tests/unit/__init__.py

@@ -0,0 +1 @@
+# Unit tests - mocked dependencies, runs in CI