fix(flow-chat): stabilize long-session turn navigation

[limityan]

Summary

This PR fixes the FlowChat scroll ownership issues reported in #1381 for long sessions and header turn navigation.

The root problem was that multiple layers were trying to own the same navigation outcome:

• the header treated a user turn selection as UI state but could not tell whether the list accepted it
• the container kept retrying header turn pins after the virtual list had already accepted responsibility
• the virtual list returned only a boolean, so callers could not distinguish immediate settlement from deferred virtualized settling
• explicit upward user intent could still be swallowed while an auto-follow arm was protected by the programmatic scroll guard

The updated behavior separates those responsibilities:

• the header closes the turn list only when the selection is immediately settled; rejected or internally pending selections keep the chooser open instead of producing false success
• the container sends a header turn pin intent once and stops retrying once the list owns a pending request
• the virtual list exposes detailed turn-pin status while preserving the existing boolean API for older callers
• explicit upward scroll intent cancels both active follow-output and armed auto-follow, so streaming/tool/background updates do not reclaim the viewport after the user starts reading history
• the long-session interaction matrix now includes real turn-navigation coverage, and the release turn-navigation spec asserts prompt panel closure after accepted selection

Validation

• vitest run src/flow_chat/components/modern/FlowChatHeader.test.tsx src/flow_chat/components/modern/ModernFlowChatContainer.history-state.test.tsx src/flow_chat/components/modern/useFlowChatFollowOutput.test.tsx src/flow_chat/components/modern/VirtualMessageList.session-boundary.test.tsx
• tsc --noEmit in src/web-ui
• node --test tests/e2e/scripts/perf-coverage-contract.test.mjs
• node tests/e2e/scripts/run-long-session-interaction-matrix.mjs --dry-run --profile scroll
• git diff --check

Fixes #1381