Inbox hydration hardening
- Defers the desktop selected-thread
react-virtuosorenderer until after the first client mount, so server-rendered/inboxmarkup and the initial client paint stay identical. - Keeps the fast virtualized renderer after hydration for long-running operator sessions, preserving attribution cards, date separators, media groups, lightbox support, and Flow response rows through the same render-item path.
- Adds regression coverage for the mount gate to prevent future hydration mismatches from pre-hydration virtualization.