Fix collapsed preview overflow render loop

[SawyerHood]

Summary

• keep expandable collapsed-preview continuation markers mounted so their width remains stable
• hide the manual marker when native truncation is active instead of removing it from layout
• add regression coverage for the ResizeObserver fit/overflow feedback loop

Root cause

A collapsed child-thread preview could fit without its manual ... marker but overflow after the marker was mounted. The overflow measurement then removed the marker, which made the preview fit again and caused an infinite ResizeObserver/render loop.

Browser profiling on the affected workspace measured 364 DOM mutations and 4.62 seconds of style recalculation over five seconds at roughly 18 FPS. Reserving the marker width reduced that sample to zero mutations, 0.033 seconds of style recalculation, and 60 FPS.

Validation

• pnpm exec turbo run typecheck --filter=@bb/app
• pnpm exec turbo run test --filter=@bb/app -- --run src/components/thread/timeline/GeneratedConversationMessage.test.tsx
• pnpm exec turbo run test --filter=@bb/app --force (241 files, 1,601 tests)
• local managed-app browser smoke test with no console or page errors