fix(flow-chat): stabilize streamed layout transitions

[wsp1911]

Summary

• Retain semantic collapse anchors while tool cards animate and Virtuoso publishes delayed height measurements.
• Reconcile sticky-pin, tail-follow, and collapse reservations without exposing transient scroll-range loss.
• Move runtime wait indicators out of modelRounds.items into fixed-height, session-scoped UI slots.
• Reuse the existing main Footer space and align BTW/Subagent status slots with their local content.
• Expand regression coverage and document the resulting scroll-stability model.

Fixes #

Type and Areas

Type:

Regression fix / UI/UX / refactor / test / docs

Areas:

Web UI, FlowChat virtualization, viewport coordination, runtime status

Motivation / Impact

Streaming tool-card collapses and delayed Virtuoso compensation could cause
visible flashes, downward jumps, lost pinned positions, or excess tail space.

Runtime status was also represented as a pseudo FlowItem, so showing or
removing it changed the virtualized list structure and triggered additional
measurement and compensation work.

This change preserves semantic anchors through delayed layout settlement,
bounds pin-owned tail space, and renders runtime status in fixed slots whose
visibility does not affect list height.

Verification

• pnpm run type-check:web — passed
• pnpm run lint:web — passed
• Focused Vitest run covering runtime status, VirtualMessageList, BTW,
  Subagent projection, persistence, dispatch, text chunks, and FlowTextBlock
  — 9 files and 108 tests passed
• Follow-up BTW and runtime slot Vitest run — 2 files and 33 tests passed
• git diff --check — passed
• Interactive validation was not run as part of this final packaging step.

Reviewer Notes

• RuntimeStatusSlot occupies the first 24px of the existing main Footer;
  it does not increase the Footer's total height.
• Runtime status is transient Zustand state keyed by session, turn, and round.
  It is no longer persisted, exported, or included in assistant output.
• Sticky-pin reservation is capped at one viewport. Collapse compensation is
  intentionally not capped because large or cumulative collapses may require
  more range to preserve a semantic anchor.
• Temporary tail reservation during delayed Virtuoso settlement is expected
  and drains through measured growth, navigation, or quiet settlement.

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.