fix(editor): keep selections whole over line backgrounds and indent guides (#1568)

* 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

src/features/notebook/CellEditor.tsx

@@ -13,6 +13,7 @@ import { bracketMatching, indentOnInput, syntaxHighlighting } from "@codemirror/
 import { openideaTheme } from "@/editor/core/cm/theme";
 import { openideaHighlightStyle } from "@/editor/core/cm/highlight";
 import { selectionMarks } from "@/editor/core/cm/selectionMarks";
+import { seamlessSelection } from "@/editor/core/cm/selectionLayer";
 import { findLanguageDescription, loadSupport } from "@/editor/core/cm/languages";
 import { tx } from "@/platform/i18n";
 import type { NotebookSession } from "./session";
@@ -45,6 +46,7 @@ export function CellEditor({
       lineNumbers(),
       drawSelection(),
       selectionMarks,
+      seamlessSelection(),
       highlightActiveLine(),
       highlightSpecialChars(),
       bracketMatching(),