* fix(editor): keep selections whole over line backgrounds and indent guides
Multi-line selections could show a 1px seam between their stacked
rectangles, indent guides showed as short dashes inside a selection,
and a breakpoint or execution line hid the selection past its text.
- Snap selection rectangle edges to device pixels so stacked pieces
share one edge.
- Paint caret-row, breakpoint and execution line backgrounds on a
pseudo-element below the selection layer.
- Draw indent guides on a pseudo-element above the text.
* test(e2e): check the visible selection layer in the theme flow