gh-142927: Fix inverted flamegraph width (#148568)

Fix inverted flamegraph width

The inverted view used thread presence as a proxy for self time.
This missed self samples on C-level wrapper frames like _run_code,
where the node's thread always appears in its children too. Those
samples were silently dropped, causing the chart to render narrower
than full width. Now uses the explicit self field on each node
instead of the thread heuristic.