fix(flow-chat): reconcile pinned viewport reservations

[wsp1911]

Summary

• Replace broad collapse-reservation retention with explicit settlement strategies based on the semantic viewport owner.
• Reconcile provisional collapse space into a freshly measured sticky-pin reservation instead of accumulating full-card estimates.
• Enter tail-follow immediately when measured content growth exhausts the remaining pin floor.
• Preserve the quiet-settlement behavior for sub-threshold growth and defer threshold settlement while a collapse transaction is active.
• Add log-derived regression coverage and document the updated scroll-stability invariants.

Type and Areas

Type:

• Regression fix
• UI/UX
• Test
• Docs

Areas:

• Web UI
• Flow Chat

Motivation / Impact

Pinned turns could remain outside tail-follow after output had reached the viewport bottom because provisional collapse space accumulated or sticky-pin growth repeatedly reset a 300 ms quiet timer.

This change assigns reservation settlement to the active viewport owner and treats complete pin-floor consumption as an immediate handoff boundary. Users should enter continuous tail-follow when output reaches the bottom without waiting for the stream to end or seeing a delayed catch-up jump.

Verification

• pnpm run type-check:web - passed
• pnpm run lint:web - passed
• pnpm --dir src/web-ui run test:run src/flow_chat/components/modern/VirtualMessageList.session-boundary.test.tsx src/flow_chat/components/modern/FlowChatViewportCoordinator.test.ts - 58 tests passed
• git diff --check - passed
• Post-fix runtime retest is pending.

Reviewer Notes

• The strict handoff condition remains unchanged: pin and collapse reservations must both be empty with no active collapse intent.
• The existing Write card lifecycle behavior is unchanged.
• FlowChat diagnostics remain behind the existing diagnostics switch.
• The settlement strategy and log-derived geometry cases are covered by focused tests.

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.