fix: trim unused ImGui build groups

[hcoona]

Summary

Trim unused ImGui build groups so UI caches do not retain stale row objects.

Detailed explanation

The ImGui build cache can retain BuildGroup entries after a later build uses fewer groups than a previous one. Those unused cached groups may continue to hold references to old UI model objects, such as rows from a table or page that is no longer displayed.

Example UI steps that trigger the issue

1. Open a view backed by a dynamic list or table.
2. Render a larger set of rows or groups.
3. Switch pages, switch projects, or otherwise render a smaller set of groups in the same cached UI structure.

Expected behavior

After a complete build, cache entries that were not used by the current build should be released so they no longer retain stale objects.

Actual behavior

Unused BuildGroup entries can remain in the cache and continue to reference objects from the previous build.

Fix

The build cache now trims unused build groups after a complete build. This keeps cache reuse for active groups while releasing stale groups that are no longer part of the current UI tree.