fix(flow-chat): smooth streamed line growth

[bobleer]

Summary

• stop treating streamed scrollHeight growth as a viewport resize that must snap to the physical bottom
• replace observer-triggered bottom snaps with a retargetable RAF tail-follow step, so line wraps settle across frames
• preserve user-scroll ownership, collapse protection, and reduced-motion behavior
• document the ownership rule and add regression coverage for line-height growth

Verification

• pnpm --dir src/web-ui run test:run src/flow_chat/components/modern/useFlowChatFollowOutput.test.tsx src/flow_chat/components/modern/VirtualMessageList.session-boundary.test.tsx src/flow_chat/components/modern/VirtualMessageList.layout.test.ts src/flow_chat/hooks/useTypewriter.test.ts (81 passed)
• pnpm run type-check:web
• targeted ESLint on changed TypeScript files (0 errors)

Testing notes

AI-assisted change. Focused automated tests and type checking passed. The standalone Web shell loaded successfully, but a full live-model desktop stream was not available because browser-only development mode has no Rust/WebSocket backend.