fix(diagnostics): avoid duplicate child stat updates

[Ninzinhu]

Summary\n\nFixes duplicate child statistic updates in StatsProvider when more than one stats listener is registered.\n\nChild-stat recursion previously ran inside the listener loop. With N registered listeners, each child was recursively processed N times and broadcast to all N listeners, producing N² child update events instead of N.\n\n## Changes\n\n- Move child-stat recursion outside the listener iteration.\n- Preserve parent and aggregate-stat notification behavior.\n- Add a regression test with two listeners and a nested child statistic.\n\n## Validation\n\n-

pm run check-types\n-
pm run lint (one pre-existing warning)\n-
pm run test -- --run (35 passed)\n-
pm run build:all\n\n## Before / after\n\nWith two listeners and one child statistic, the previous implementation delivered four child events. This change delivers two: one per listener.