normalize list/array handling in data classes

Establish one invariant across the Drawable hierarchy: x is always a
numpy ndarray, y is always a list of per-channel ndarrays, and empty
data is the same types with zero length.

- Drawable: x starts as empty ndarray; min/max helpers return 0 on
  empty data instead of raising; drop contradictory isinstance(self.y,
  list) branches
- ADC_Result/Accumulation: empty constructors build ndarray x; full
  constructors coerce x/y/y_square with numpy.asarray; Accumulation
  get_ydata/get_yerr return empty ndarrays and no longer crash on
  empty statistics-less accumulations
- MeasurementResult: fix get_lineplotdata to return the accumulated
  means (previously the never-populated self.y, so the GUI line plot
  could never draw); remove dead float/int branches in CSV and HDF
  writers
- Error_Result: get_xdata/get_ydata return ndarrays

Add unittest coverage for the invariant, empty-state accessors,
constructor coercion, line plot data, CSV output and HDF round-trip.

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

tests/test_MeasurementResult.py

@@ -1,8 +1,12 @@
+import io
+import os
+import tempfile
 import unittest
 
 import numpy as np
+import tables
 
-from damaris.data.MeasurementResult import MeasurementResult
+from damaris.data.MeasurementResult import MeasurementResult, read_from_hdf
 
 
 class TestMeasurementResult(unittest.TestCase):
@@ -30,6 +34,53 @@ class TestMeasurementResult(unittest.TestCase):
         mr = MeasurementResult("q")
         self.assertRaises(Exception, mr.__add__, 1)
 
+    def test_get_lineplotdata_returns_means(self):
+        """get_lineplotdata must return the accumulated means, not self.y."""
+        mr = MeasurementResult("test_quantity")
+        for i in range(3):
+            mr[float(i)] = float(i * i)
+        k, v = mr.get_lineplotdata()
+        self.assertIsInstance(k, np.ndarray)
+        self.assertIsInstance(v, np.ndarray)
+        self.assertEqual(k.shape, v.shape)
+        np.testing.assert_array_equal(v, mr.get_ydata())
+        np.testing.assert_array_equal(v, [0.0, 1.0, 4.0])
+
+    def test_get_lineplotdata_empty(self):
+        """get_lineplotdata of an empty result must return empty arrays."""
+        mr = MeasurementResult("q")
+        k, v = mr.get_lineplotdata()
+        self.assertEqual(k.shape[0], 0)
+        self.assertEqual(v.shape[0], 0)
+
+    def test_write_to_csv(self):
+        mr = MeasurementResult("q")
+        mr[1.0] = 5.0
+        mr[2.0] = 7.0
+        buf = io.StringIO()
+        mr.write_to_csv(buf)
+        lines = buf.getvalue().splitlines()
+        self.assertEqual(lines[0], "# quantity:q")
+        self.assertEqual(lines[1], "# x y ysigma n")
+        self.assertEqual(len(lines), 4)
+        self.assertEqual(lines[2].split()[3], "1")
+        self.assertEqual(lines[3].split()[3], "1")
+
+    def test_write_to_hdf_roundtrip(self):
+        mr = MeasurementResult("q")
+        mr[1.0] = 5.0
+        mr[2.0] = 7.0
+        with tempfile.TemporaryDirectory() as tmpdir:
+            path = os.path.join(tmpdir, "test.h5")
+            with tables.open_file(path, mode="w") as h5file:
+                mr.write_to_hdf(h5file, "/", "mr", "test result")
+            with tables.open_file(path, mode="r") as h5file:
+                mr2 = read_from_hdf(h5file.get_node("/mr"))
+        self.assertIsNotNone(mr2)
+        self.assertEqual(mr2.quantity_name, "q")
+        np.testing.assert_array_equal(mr2.get_xdata(), mr.get_xdata())
+        np.testing.assert_array_equal(mr2.get_ydata(), mr.get_ydata())
+
 
 if __name__ == "__main__":
     unittest.main()