Linearize UI reconciliation

[VolkerChristian]

Summary

• rebuild thread ordering in one pass and aggregate pending-request counts once
• precompute thread-row positions and update only rows whose positions changed
• retain explicit conversation section/card order and eliminate repeated Qt layout searches
• consolidate reconciliation passes while preserving widget identity, scroll anchors, folds, selection, and command output
• use natural viewport boundaries for bottom-edge card folding and reveal an expanded card above the composer

Validation

• full test suite: 7/7 passed
• direct CodexUI validation: navigation, streamed command output, fold/unfold during a running command, completion, and bottom-edge folding with the composer boundary