Add tests for inset_element

Covers the inset_element work that's accumulated over the branch:

- `TestInsetAlignTo` — panel / plot / full host regions.
- `TestNestedOnTop` — `on_top` toggled on a deeply nested inset.
- `TestQuadrantInsets` — four-in-a-grid via separate insets vs. one
  composition inset.
- `TestPropagateTheme` — `&` / `*` broadcast from a host into its
  insets (including Compose and nested ggplot insets) and the
  TypeError guards on bare-ggplot operands.
- `TestImageInset` — aspect-fit + anchor placement, themed background
  wrapping the user bbox, ndarray/PIL parity, standalone render,
  and construction-time validation.

Author: has2k1