Feature/list viewer 1399

[Fred-Wu]

Closes #1399
Closes #1687
Closes #1576

Summary

Adds a dedicated viewer for R lists and other list-like objects, allowing their contents to be inspected without expanding the entire object as text.

List viewer

• View() now opens lists, environments, and S4 objects in a dedicated list viewer.

• Each child is shown with its name/index and a compact str() summary.

• Viewable children have a View button and can be opened recursively:

  • data frames and matrices open in the Data Viewer;
  • nested lists, environments, and S4 objects open in another List Viewer;
  • other objects continue to use the existing object viewer.

• Nested viewer titles preserve the R access path, using $name, [[index]], and @slot as appropriate.

• Repeated View() calls for the same titled object reuse the existing viewer instead of opening duplicate panels.

Lazy loading

• List contents are retrieved from the R session on demand rather than serialising the complete object when the viewer opens, similar to the implementation for the workspace viewer.

Tests

Adds regression coverage for:

• viewable structured and non-structured workspace children;
• paged list inspection and stable child indices;
• avoiding evaluation of unrequested and active bindings.

[Fred-Wu]

I will leave this to 3.x milestone.

Here is the current behavior.

Screen.Recording.2026-09-20.at.10.20.06.pm.mov

The View button is also added for nested objects in the Workspace Viewer after expansion.