Second step of the virtualization plan. No virtualization yet. This PR
makes IntersectionObserver-based visibility tracking correct inside
scroll containers (sidebar, bottom-sheet), which is a prerequisite for
virtualizing the timeline.
- Introduces `TimelineViewport` — a grouped contract passed as a single
`viewport` prop on `ActivityTimeline`. Holds `scrollElementRef`,
`observerRoot`, `scrollMargin`, and `virtualizationEnabled`.
- `MessageRowWithObserver` and `LeadThoughtsGroupRow` now create their
`IntersectionObserver` with `root = observerRoot?.current ?? null`
instead of defaulting to the document viewport. Unread marking now
fires when rows enter their real scroll parent.
- `MessagesPanel` resolves the active scroll owner from `position`
(inline from parent ref, sidebar from `sidebarScrollRef`, bottom-sheet
from `bottomSheetScrollRef`) and passes it into ActivityTimeline.
- Tests: stubs `IntersectionObserver` to capture `options.root` and
asserts null when no viewport is passed, and the provided element when
`viewport.observerRoot` is set.
`scrollMargin` and `virtualizationEnabled` are included in the contract
but not consumed yet — they land in follow-up PRs (#4/#5).
d4f518e8c5