fix y_square latent bug: keep it always a list of per-channel arrays

Accumulation.__neg__ passed y_2 = numpy.array(self.y_square), storing a
2D ndarray. A later __iadd__ on an accumulation with ndarray y_square
crashed with AttributeError at the y_square.append sites (992, 1064).

- __neg__ now passes a per-channel copy (values unchanged: sums of
  squares are invariant under negation)
- __init__ normalizes ndarray y_2 to a list, so external callers cannot
  reintroduce the bug
- y_square annotated as _YChannels

Clears the remaining pyrefly errors at Accumulation.py:992 and :1064
(74 -> 72). Unit tests reproduce both crashes and pass with the fix.

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

src/damaris/data/Accumulation.py

@@ -77,7 +77,8 @@ class Accumulation(Errorable, Drawable, DamarisFFT, Signalpath):
         self.use_error = error
         if self.uses_statistics():
             if (y_2 is not None):
-                self.y_square = y_2
+                # y_square is always kept as a list of per-channel arrays
+                self.y_square: _YChannels = y_2 if isinstance(y_2, list) else list(y_2)
             elif (y_2 is None) :
                 self.y_square = []
             else:
@@ -1147,7 +1148,8 @@ class Accumulation(Errorable, Drawable, DamarisFFT, Signalpath):
             tmp_y.append(numpy.array(-self.y[i], dtype="float32"))
 
         if self.uses_statistics():
-            r = Accumulation(x = numpy.array(self.x, dtype="float32"), y = tmp_y, y_2 = numpy.array(self.y_square), n = self.n, index = self.index, sampl_freq = self.sampling_rate, error = True)
+            tmp_y_square = [numpy.array(sq) for sq in self.y_square]
+            r = Accumulation(x = numpy.array(self.x, dtype="float32"), y = tmp_y, y_2 = tmp_y_square, n = self.n, index = self.index, sampl_freq = self.sampling_rate, error = True)
         else:
             r = Accumulation(x = numpy.array(self.x, dtype="float32"), y = tmp_y, n = self.n, index = self.index, sampl_freq = self.sampling_rate, error = False)
         self._copy_provenance(r)