Make ImageArray a descriptor to enforce ImageFile.image type invariant

[Copilot]

ImageFile.image was a plain @property with no type enforcement at the class level. This mirrors the DataArray descriptor pattern (PR #32) to ImageArray/ImageFile.

Changes

• ImageArray — descriptor protocol: Added __set_name__, __get__, __set__ so any ImageArray instance can serve as a data descriptor. The setter preserves filename/metadata from the previous image and handles the in-place stack-update path.

• ImageFile — replace @property with class-level descriptor:

  # Before
  @property
  def image(self): return self._image
  @image.setter
  def image(self, v): ...  # 12 lines of conversion logic
  
  # After
  image = ImageArray()
  """ImageArray descriptor that enforces the image attribute is always an ImageArray instance."""

• KerrImageFile — fix subclass override: @ImageFile.image.getter / @ImageFile.image.setter are property-only patterns that broke once image became a descriptor. Replaced with a self-contained @property / @image.setter pair that overrides the descriptor in the subclass.

• Tests: Added test_image_descriptor covering class-level type assertion, numpy→ImageArray coercion, metadata preservation, and filename preservation across image replacement.

---

⌨️ Start Copilot coding agent tasks without leaving your editor — available in VS Code, Visual Studio, JetBrains IDEs and Eclipse.