fix(flowchat): fix arbitrary turn navigation across renderer handoff

[wsp1911]

Summary

Fix arbitrary FlowChat turn navigation during and after streaming, including navigation across the static-history to Virtuoso renderer handoff.

Type and Areas

Type:

• bug fix
• regression fix
• UI/UX
• docs
• test

Areas:

• Web UI
• FlowChat

Motivation / Impact

Previously, arbitrary turn navigation could remain pending, jump back to the bottom, or stop working after a new turn switched rendering modes.

This change:

• Preserves the existing footer and pin range while distant targets materialize.
• Uses local Virtuoso data indices for navigation APIs.
• Resolves pending navigation through rangeChanged and bounded retries.
• Carries pending targets through the static-history to Virtuoso handoff.
• Aligns the target user message to the shared top offset.
• Adds cancellation and timeout recovery without silently falling to the bottom.
• Rebinds ScrollAnchor to the concrete scroller element.
• Adds regression tests and scroll stability documentation.

Verification

• pnpm run type-check:web
• pnpm --dir src/web-ui run lint
• pnpm --dir src/web-ui run test:run src/flow_chat/components/modern/VirtualMessageList.session-boundary.test.tsx src/flow_chat/components/modern/ScrollAnchor.test.tsx
• git diff --check
• Focused tests: 51 passed
• Manual reproduction confirmed that arbitrary turn navigation works during and after streaming.

Reviewer Notes

• Virtuoso navigation now uses local data indices; firstItemIndex remains only a rendering coordinate.
• Navigation follows a materialize-then-align transaction.
• The existing physical range is preserved until the new target is rendered and aligned.
• rangeChanged participates in target materialization, with bounded RAF retries as fallback.
• The root flowchat-turn-navigation-fix-plan.md file is an explicitly requested durable diagnostic and design artifact.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.