test: add pytest dev dep + tests scaffold

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

tests/__init__.py

@@ -0,0 +1 @@
+

tests/conftest.py

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+"""Make `presa_mano.*` importable in tests without installing the package."""
+from __future__ import annotations
+
+import sys
+from pathlib import Path
+
+_SRC = Path(__file__).resolve().parent.parent / "src"
+if str(_SRC) not in sys.path:
+    sys.path.insert(0, str(_SRC))