fix(core): replace deprecated ast.Num visitor in FPS parser

ast.Num/node.n deprecated since Python 3.8; NodeVisitor falls back to visit_Num with a DeprecationWarning per dispatch. Under -W error this surfaced as a misleading fps ValueError in Video.init. Replace with visit_Constant, reject non-numeric constants, and pin parse results for int/fraction/float inputs in a regression test.

tests/core/test_fps.py

@@ -0,0 +1,29 @@
+import warnings
+
+import pytest
+
+from unshackle.core.utilities import FPS
+
+
+@pytest.mark.parametrize(
+    ("expr", "expected"),
+    [
+        ("24", 24),
+        ("23.976", pytest.approx(23.976)),
+        ("30000/1001", pytest.approx(29.97, abs=0.001)),
+    ],
+)
+def test_parse_pins_results_without_deprecation_warnings(expr: str, expected: object) -> None:
+    with warnings.catch_warnings():
+        warnings.simplefilter("error", DeprecationWarning)
+        assert FPS.parse(expr) == expected
+
+
+def test_parse_rejects_non_numeric_constant() -> None:
+    with pytest.raises(ValueError, match="Invalid fps value"):
+        FPS.parse("'24'")
+
+
+def test_parse_rejects_non_division_operation() -> None:
+    with pytest.raises(ValueError, match="Invalid operation"):
+        FPS.parse("24+1")