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

e2e/themes.spec.ts

@@ -111,7 +111,7 @@ for (const palette of palettes) {
       );
     await editor.locator(".cm-content").click();
     await page.keyboard.press("Meta+a");
-    await expect(editor.locator(".cm-selectionBackground").first()).toBeVisible();
+    await expect(editor.locator(".cm-selectionBackground:visible").first()).toBeVisible();
     await page.keyboard.press("ArrowRight");
     await page.keyboard.press("Meta+f");
     await page.getByRole("textbox", { name: "Find", exact: true }).fill("Order");